Merge from gnulib

This incorporates:
2017-03-14 snippets: move unadjusted snippet sources to lib
2017-03-14 gnulib-tool: fix typo in comment output
2017-03-14 snippets: work around GNU Make 3.82 VPATH
2017-03-13 gnulib-tool: minor --gnu-make fixups
2017-03-12 gnulib-tool: new option --gnu-make
* .gitignore: Remove lib/arg-nonnull.h, lib/c++defs.h,
lib/warn-on-use.h.  Change exception from
build-aux/snippet/_Noreturn.h to lib/_Noreturn.h.
* admin/authors.el (authors-renamed-files-regexps):
* admin/notes/copyright, make-dist:
The snippet files moved from build-aux/snippet to lib.
* lib/_Noreturn.h: Rename from build-aux/snippet/_Noreturn.h.
* lib/arg-nonnull.h: Rename from build-aux/snippet/arg-nonnull.h.
* lib/c++defs.h: Rename from build-aux/snippet/c++defs.h.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/warn-on-use.h: Rename from build-aux/snippet/warn-on-use.h.

5 files changed, 480 insertions, 51 deletions

diff --git a/lib/_Noreturn.h b/lib/_Noreturn.h
new file mode 100644
index 00000000000..c44ad89b7c0
--- /dev/null
+++ b/lib/_Noreturn.h
@@ -0,0 +1,10 @@
+#if !defined _Noreturn && __STDC_VERSION__ < 201112
+# if (3 <= __GNUC__ || (__GNUC__ == 2 && 8 <= __GNUC_MINOR__) \
+      || 0x5110 <= __SUNPRO_C)
+#  define _Noreturn __attribute__ ((__noreturn__))
+# elif 1200 <= _MSC_VER
+#  define _Noreturn __declspec (noreturn)
+# else
+#  define _Noreturn
+# endif
+#endif
diff --git a/lib/arg-nonnull.h b/lib/arg-nonnull.h
new file mode 100644
index 00000000000..1e62cc89827
--- /dev/null
+++ b/lib/arg-nonnull.h
@@ -0,0 +1,26 @@
+/* A C macro for declaring that specific arguments must not be NULL.
+   Copyright (C) 2009-2017 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published
+   by the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* _GL_ARG_NONNULL((n,...,m)) tells the compiler and static analyzer tools
+   that the values passed as arguments n, ..., m must be non-NULL pointers.
+   n = 1 stands for the first argument, n = 2 for the second argument etc.  */
+#ifndef _GL_ARG_NONNULL
+# if (__GNUC__ == 3 && __GNUC_MINOR__ >= 3) || __GNUC__ > 3
+#  define _GL_ARG_NONNULL(params) __attribute__ ((__nonnull__ params))
+# else
+#  define _GL_ARG_NONNULL(params)
+# endif
+#endif
diff --git a/lib/c++defs.h b/lib/c++defs.h
new file mode 100644
index 00000000000..f03f3591c35
--- /dev/null
+++ b/lib/c++defs.h
@@ -0,0 +1,316 @@
+/* C++ compatible function declaration macros.
+   Copyright (C) 2010-2017 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published
+   by the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef _GL_CXXDEFS_H
+#define _GL_CXXDEFS_H
+
+/* Begin/end the GNULIB_NAMESPACE namespace.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_BEGIN_NAMESPACE namespace GNULIB_NAMESPACE {
+# define _GL_END_NAMESPACE }
+#else
+# define _GL_BEGIN_NAMESPACE
+# define _GL_END_NAMESPACE
+#endif
+
+/* The three most frequent use cases of these macros are:
+
+   * For providing a substitute for a function that is missing on some
+     platforms, but is declared and works fine on the platforms on which
+     it exists:
+
+       #if @GNULIB_FOO@
+       # if !@HAVE_FOO@
+       _GL_FUNCDECL_SYS (foo, ...);
+       # endif
+       _GL_CXXALIAS_SYS (foo, ...);
+       _GL_CXXALIASWARN (foo);
+       #elif defined GNULIB_POSIXCHECK
+       ...
+       #endif
+
+   * For providing a replacement for a function that exists on all platforms,
+     but is broken/insufficient and needs to be replaced on some platforms:
+
+       #if @GNULIB_FOO@
+       # if @REPLACE_FOO@
+       #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
+       #   undef foo
+       #   define foo rpl_foo
+       #  endif
+       _GL_FUNCDECL_RPL (foo, ...);
+       _GL_CXXALIAS_RPL (foo, ...);
+       # else
+       _GL_CXXALIAS_SYS (foo, ...);
+       # endif
+       _GL_CXXALIASWARN (foo);
+       #elif defined GNULIB_POSIXCHECK
+       ...
+       #endif
+
+   * For providing a replacement for a function that exists on some platforms
+     but is broken/insufficient and needs to be replaced on some of them and
+     is additionally either missing or undeclared on some other platforms:
+
+       #if @GNULIB_FOO@
+       # if @REPLACE_FOO@
+       #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
+       #   undef foo
+       #   define foo rpl_foo
+       #  endif
+       _GL_FUNCDECL_RPL (foo, ...);
+       _GL_CXXALIAS_RPL (foo, ...);
+       # else
+       #  if !@HAVE_FOO@   or   if !@HAVE_DECL_FOO@
+       _GL_FUNCDECL_SYS (foo, ...);
+       #  endif
+       _GL_CXXALIAS_SYS (foo, ...);
+       # endif
+       _GL_CXXALIASWARN (foo);
+       #elif defined GNULIB_POSIXCHECK
+       ...
+       #endif
+*/
+
+/* _GL_EXTERN_C declaration;
+   performs the declaration with C linkage.  */
+#if defined __cplusplus
+# define _GL_EXTERN_C extern "C"
+#else
+# define _GL_EXTERN_C extern
+#endif
+
+/* _GL_FUNCDECL_RPL (func, rettype, parameters_and_attributes);
+   declares a replacement function, named rpl_func, with the given prototype,
+   consisting of return type, parameters, and attributes.
+   Example:
+     _GL_FUNCDECL_RPL (open, int, (const char *filename, int flags, ...)
+                                  _GL_ARG_NONNULL ((1)));
+ */
+#define _GL_FUNCDECL_RPL(func,rettype,parameters_and_attributes) \
+  _GL_FUNCDECL_RPL_1 (rpl_##func, rettype, parameters_and_attributes)
+#define _GL_FUNCDECL_RPL_1(rpl_func,rettype,parameters_and_attributes) \
+  _GL_EXTERN_C rettype rpl_func parameters_and_attributes
+
+/* _GL_FUNCDECL_SYS (func, rettype, parameters_and_attributes);
+   declares the system function, named func, with the given prototype,
+   consisting of return type, parameters, and attributes.
+   Example:
+     _GL_FUNCDECL_SYS (open, int, (const char *filename, int flags, ...)
+                                  _GL_ARG_NONNULL ((1)));
+ */
+#define _GL_FUNCDECL_SYS(func,rettype,parameters_and_attributes) \
+  _GL_EXTERN_C rettype func parameters_and_attributes
+
+/* _GL_CXXALIAS_RPL (func, rettype, parameters);
+   declares a C++ alias called GNULIB_NAMESPACE::func
+   that redirects to rpl_func, if GNULIB_NAMESPACE is defined.
+   Example:
+     _GL_CXXALIAS_RPL (open, int, (const char *filename, int flags, ...));
+
+   Wrapping rpl_func in an object with an inline conversion operator
+   avoids a reference to rpl_func unless GNULIB_NAMESPACE::func is
+   actually used in the program.  */
+#define _GL_CXXALIAS_RPL(func,rettype,parameters) \
+  _GL_CXXALIAS_RPL_1 (func, rpl_##func, rettype, parameters)
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIAS_RPL_1(func,rpl_func,rettype,parameters) \
+    namespace GNULIB_NAMESPACE                                \
+    {                                                         \
+      static const struct _gl_ ## func ## _wrapper            \
+      {                                                       \
+        typedef rettype (*type) parameters;                   \
+                                                              \
+        inline operator type () const                         \
+        {                                                     \
+          return ::rpl_func;                                  \
+        }                                                     \
+      } func = {};                                            \
+    }                                                         \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#else
+# define _GL_CXXALIAS_RPL_1(func,rpl_func,rettype,parameters) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIAS_RPL_CAST_1 (func, rpl_func, rettype, parameters);
+   is like  _GL_CXXALIAS_RPL_1 (func, rpl_func, rettype, parameters);
+   except that the C function rpl_func may have a slightly different
+   declaration.  A cast is used to silence the "invalid conversion" error
+   that would otherwise occur.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIAS_RPL_CAST_1(func,rpl_func,rettype,parameters) \
+    namespace GNULIB_NAMESPACE                                     \
+    {                                                              \
+      static const struct _gl_ ## func ## _wrapper                 \
+      {                                                            \
+        typedef rettype (*type) parameters;                        \
+                                                                   \
+        inline operator type () const                              \
+        {                                                          \
+          return reinterpret_cast<type>(::rpl_func);               \
+        }                                                          \
+      } func = {};                                                 \
+    }                                                              \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#else
+# define _GL_CXXALIAS_RPL_CAST_1(func,rpl_func,rettype,parameters) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIAS_SYS (func, rettype, parameters);
+   declares a C++ alias called GNULIB_NAMESPACE::func
+   that redirects to the system provided function func, if GNULIB_NAMESPACE
+   is defined.
+   Example:
+     _GL_CXXALIAS_SYS (open, int, (const char *filename, int flags, ...));
+
+   Wrapping func in an object with an inline conversion operator
+   avoids a reference to func unless GNULIB_NAMESPACE::func is
+   actually used in the program.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIAS_SYS(func,rettype,parameters)            \
+    namespace GNULIB_NAMESPACE                                \
+    {                                                         \
+      static const struct _gl_ ## func ## _wrapper            \
+      {                                                       \
+        typedef rettype (*type) parameters;                   \
+                                                              \
+        inline operator type () const                         \
+        {                                                     \
+          return ::func;                                      \
+        }                                                     \
+      } func = {};                                            \
+    }                                                         \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#else
+# define _GL_CXXALIAS_SYS(func,rettype,parameters) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIAS_SYS_CAST (func, rettype, parameters);
+   is like  _GL_CXXALIAS_SYS (func, rettype, parameters);
+   except that the C function func may have a slightly different declaration.
+   A cast is used to silence the "invalid conversion" error that would
+   otherwise occur.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIAS_SYS_CAST(func,rettype,parameters) \
+    namespace GNULIB_NAMESPACE                          \
+    {                                                   \
+      static const struct _gl_ ## func ## _wrapper      \
+      {                                                 \
+        typedef rettype (*type) parameters;             \
+                                                        \
+        inline operator type () const                   \
+        {                                               \
+          return reinterpret_cast<type>(::func);        \
+        }                                               \
+      } func = {};                                      \
+    }                                                   \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#else
+# define _GL_CXXALIAS_SYS_CAST(func,rettype,parameters) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIAS_SYS_CAST2 (func, rettype, parameters, rettype2, parameters2);
+   is like  _GL_CXXALIAS_SYS (func, rettype, parameters);
+   except that the C function is picked among a set of overloaded functions,
+   namely the one with rettype2 and parameters2.  Two consecutive casts
+   are used to silence the "cannot find a match" and "invalid conversion"
+   errors that would otherwise occur.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+  /* The outer cast must be a reinterpret_cast.
+     The inner cast: When the function is defined as a set of overloaded
+     functions, it works as a static_cast<>, choosing the designated variant.
+     When the function is defined as a single variant, it works as a
+     reinterpret_cast<>. The parenthesized cast syntax works both ways.  */
+# define _GL_CXXALIAS_SYS_CAST2(func,rettype,parameters,rettype2,parameters2) \
+    namespace GNULIB_NAMESPACE                                                \
+    {                                                                         \
+      static const struct _gl_ ## func ## _wrapper                            \
+      {                                                                       \
+        typedef rettype (*type) parameters;                                   \
+                                                                              \
+        inline operator type () const                                         \
+        {                                                                     \
+          return reinterpret_cast<type>((rettype2 (*) parameters2)(::func));  \
+        }                                                                     \
+      } func = {};                                                            \
+    }                                                                         \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#else
+# define _GL_CXXALIAS_SYS_CAST2(func,rettype,parameters,rettype2,parameters2) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIASWARN (func);
+   causes a warning to be emitted when ::func is used but not when
+   GNULIB_NAMESPACE::func is used.  func must be defined without overloaded
+   variants.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIASWARN(func) \
+   _GL_CXXALIASWARN_1 (func, GNULIB_NAMESPACE)
+# define _GL_CXXALIASWARN_1(func,namespace) \
+   _GL_CXXALIASWARN_2 (func, namespace)
+/* To work around GCC bug <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43881>,
+   we enable the warning only when not optimizing.  */
+# if !__OPTIMIZE__
+#  define _GL_CXXALIASWARN_2(func,namespace) \
+    _GL_WARN_ON_USE (func, \
+                     "The symbol ::" #func " refers to the system function. " \
+                     "Use " #namespace "::" #func " instead.")
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+#  define _GL_CXXALIASWARN_2(func,namespace) \
+     extern __typeof__ (func) func
+# else
+#  define _GL_CXXALIASWARN_2(func,namespace) \
+     _GL_EXTERN_C int _gl_cxxalias_dummy
+# endif
+#else
+# define _GL_CXXALIASWARN(func) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+/* _GL_CXXALIASWARN1 (func, rettype, parameters_and_attributes);
+   causes a warning to be emitted when the given overloaded variant of ::func
+   is used but not when GNULIB_NAMESPACE::func is used.  */
+#if defined __cplusplus && defined GNULIB_NAMESPACE
+# define _GL_CXXALIASWARN1(func,rettype,parameters_and_attributes) \
+   _GL_CXXALIASWARN1_1 (func, rettype, parameters_and_attributes, \
+                        GNULIB_NAMESPACE)
+# define _GL_CXXALIASWARN1_1(func,rettype,parameters_and_attributes,namespace) \
+   _GL_CXXALIASWARN1_2 (func, rettype, parameters_and_attributes, namespace)
+/* To work around GCC bug <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43881>,
+   we enable the warning only when not optimizing.  */
+# if !__OPTIMIZE__
+#  define _GL_CXXALIASWARN1_2(func,rettype,parameters_and_attributes,namespace) \
+    _GL_WARN_ON_USE_CXX (func, rettype, parameters_and_attributes, \
+                         "The symbol ::" #func " refers to the system function. " \
+                         "Use " #namespace "::" #func " instead.")
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+#  define _GL_CXXALIASWARN1_2(func,rettype,parameters_and_attributes,namespace) \
+     extern __typeof__ (func) func
+# else
+#  define _GL_CXXALIASWARN1_2(func,rettype,parameters_and_attributes,namespace) \
+     _GL_EXTERN_C int _gl_cxxalias_dummy
+# endif
+#else
+# define _GL_CXXALIASWARN1(func,rettype,parameters_and_attributes) \
+    _GL_EXTERN_C int _gl_cxxalias_dummy
+#endif
+
+#endif /* _GL_CXXDEFS_H */
diff --git a/lib/gnulib.mk b/lib/gnulib.mk
index 7a0de1b440d..4109e7f6af5 100644
--- a/lib/gnulib.mk
+++ b/lib/gnulib.mk
@@ -25,6 +25,7 @@
 
 
 MOSTLYCLEANFILES += core *.stackdump
+# No GNU Make output.
 
 noinst_LIBRARIES += libgnu.a
 
@@ -873,81 +874,48 @@ EXTRA_DIST += signal.in.h
 ## begin gnulib module snippet/_Noreturn
 
 # Because this Makefile snippet defines a variable used by other
-# gnulib Makefile snippets, it must be present in all Makefile.am that
+# gnulib Makefile snippets, it must be present in all makefiles that
 # need it. This is ensured by the applicability 'all' defined above.
 
-_NORETURN_H=$(top_srcdir)/build-aux/snippet/_Noreturn.h
+_NORETURN_H=$(srcdir)/_Noreturn.h
 
-EXTRA_DIST += $(top_srcdir)/build-aux/snippet/_Noreturn.h
+EXTRA_DIST += _Noreturn.h
 
 ## end   gnulib module snippet/_Noreturn
 
 ## begin gnulib module snippet/arg-nonnull
 
-# The BUILT_SOURCES created by this Makefile snippet are not used via #include
-# statements but through direct file reference. Therefore this snippet must be
-# present in all Makefile.am that need it. This is ensured by the applicability
-# 'all' defined above.
-
-BUILT_SOURCES += arg-nonnull.h
-# The arg-nonnull.h that gets inserted into generated .h files is the same as
-# build-aux/snippet/arg-nonnull.h, except that it has the copyright header cut
-# off.
-arg-nonnull.h: $(top_srcdir)/build-aux/snippet/arg-nonnull.h
-	$(AM_V_GEN)rm -f $@-t $@ && \
-	sed -n -e '/GL_ARG_NONNULL/,$$p' \
-	  < $(top_srcdir)/build-aux/snippet/arg-nonnull.h \
-	  > $@-t && \
-	mv $@-t $@
-MOSTLYCLEANFILES += arg-nonnull.h arg-nonnull.h-t
+# Because this Makefile snippet defines a variable used by other
+# gnulib Makefile snippets, it must be present in all makefiles that
+# need it. This is ensured by the applicability 'all' defined above.
 
-ARG_NONNULL_H=arg-nonnull.h
+ARG_NONNULL_H=$(srcdir)/arg-nonnull.h
 
-EXTRA_DIST += $(top_srcdir)/build-aux/snippet/arg-nonnull.h
+EXTRA_DIST += arg-nonnull.h
 
 ## end   gnulib module snippet/arg-nonnull
 
 ## begin gnulib module snippet/c++defs
 
-# The BUILT_SOURCES created by this Makefile snippet are not used via #include
-# statements but through direct file reference. Therefore this snippet must be
-# present in all Makefile.am that need it. This is ensured by the applicability
-# 'all' defined above.
-
-BUILT_SOURCES += c++defs.h
-# The c++defs.h that gets inserted into generated .h files is the same as
-# build-aux/snippet/c++defs.h, except that it has the copyright header cut off.
-c++defs.h: $(top_srcdir)/build-aux/snippet/c++defs.h
-	$(AM_V_GEN)rm -f $@-t $@ && \
-	sed -n -e '/_GL_CXXDEFS/,$$p' \
-	  < $(top_srcdir)/build-aux/snippet/c++defs.h \
-	  > $@-t && \
-	mv $@-t $@
-MOSTLYCLEANFILES += c++defs.h c++defs.h-t
+# Because this Makefile snippet defines a variable used by other
+# gnulib Makefile snippets, it must be present in all makefiles that
+# need it. This is ensured by the applicability 'all' defined above.
 
-CXXDEFS_H=c++defs.h
+CXXDEFS_H=$(srcdir)/c++defs.h
 
-EXTRA_DIST += $(top_srcdir)/build-aux/snippet/c++defs.h
+EXTRA_DIST += c++defs.h
 
 ## end   gnulib module snippet/c++defs
 
 ## begin gnulib module snippet/warn-on-use
 
-BUILT_SOURCES += warn-on-use.h
-# The warn-on-use.h that gets inserted into generated .h files is the same as
-# build-aux/snippet/warn-on-use.h, except that it has the copyright header cut
-# off.
-warn-on-use.h: $(top_srcdir)/build-aux/snippet/warn-on-use.h
-	$(AM_V_GEN)rm -f $@-t $@ && \
-	sed -n -e '/^.ifndef/,$$p' \
-	  < $(top_srcdir)/build-aux/snippet/warn-on-use.h \
-	  > $@-t && \
-	mv $@-t $@
-MOSTLYCLEANFILES += warn-on-use.h warn-on-use.h-t
+# Because this Makefile snippet defines a variable used by other
+# gnulib Makefile snippets, it must be present in all makefiles that
+# need it. This is ensured by the applicability 'all' defined above.
 
-WARN_ON_USE_H=warn-on-use.h
+WARN_ON_USE_H=$(srcdir)/warn-on-use.h
 
-EXTRA_DIST += $(top_srcdir)/build-aux/snippet/warn-on-use.h
+EXTRA_DIST += warn-on-use.h
 
 ## end   gnulib module snippet/warn-on-use
 
diff --git a/lib/warn-on-use.h b/lib/warn-on-use.h
new file mode 100644
index 00000000000..3c0eb579fa2
--- /dev/null
+++ b/lib/warn-on-use.h
@@ -0,0 +1,109 @@
+/* A C macro for emitting warnings if a function is used.
+   Copyright (C) 2010-2017 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published
+   by the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* _GL_WARN_ON_USE (function, "literal string") issues a declaration
+   for FUNCTION which will then trigger a compiler warning containing
+   the text of "literal string" anywhere that function is called, if
+   supported by the compiler.  If the compiler does not support this
+   feature, the macro expands to an unused extern declaration.
+
+   This macro is useful for marking a function as a potential
+   portability trap, with the intent that "literal string" include
+   instructions on the replacement function that should be used
+   instead.  However, one of the reasons that a function is a
+   portability trap is if it has the wrong signature.  Declaring
+   FUNCTION with a different signature in C is a compilation error, so
+   this macro must use the same type as any existing declaration so
+   that programs that avoid the problematic FUNCTION do not fail to
+   compile merely because they included a header that poisoned the
+   function.  But this implies that _GL_WARN_ON_USE is only safe to
+   use if FUNCTION is known to already have a declaration.  Use of
+   this macro implies that there must not be any other macro hiding
+   the declaration of FUNCTION; but undefining FUNCTION first is part
+   of the poisoning process anyway (although for symbols that are
+   provided only via a macro, the result is a compilation error rather
+   than a warning containing "literal string").  Also note that in
+   C++, it is only safe to use if FUNCTION has no overloads.
+
+   For an example, it is possible to poison 'getline' by:
+   - adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
+     [getline]) in configure.ac, which potentially defines
+     HAVE_RAW_DECL_GETLINE
+   - adding this code to a header that wraps the system <stdio.h>:
+     #undef getline
+     #if HAVE_RAW_DECL_GETLINE
+     _GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
+       "not universally present; use the gnulib module getline");
+     #endif
+
+   It is not possible to directly poison global variables.  But it is
+   possible to write a wrapper accessor function, and poison that
+   (less common usage, like &environ, will cause a compilation error
+   rather than issue the nice warning, but the end result of informing
+   the developer about their portability problem is still achieved):
+   #if HAVE_RAW_DECL_ENVIRON
+   static char ***rpl_environ (void) { return &environ; }
+   _GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
+   # undef environ
+   # define environ (*rpl_environ ())
+   #endif
+   */
+#ifndef _GL_WARN_ON_USE
+
+# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
+/* A compiler attribute is available in gcc versions 4.3.0 and later.  */
+#  define _GL_WARN_ON_USE(function, message) \
+extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+/* Verify the existence of the function.  */
+#  define _GL_WARN_ON_USE(function, message) \
+extern __typeof__ (function) function
+# else /* Unsupported.  */
+#  define _GL_WARN_ON_USE(function, message) \
+_GL_WARN_EXTERN_C int _gl_warn_on_use
+# endif
+#endif
+
+/* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
+   is like _GL_WARN_ON_USE (function, "string"), except that the function is
+   declared with the given prototype, consisting of return type, parameters,
+   and attributes.
+   This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
+   not work in this case.  */
+#ifndef _GL_WARN_ON_USE_CXX
+# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+extern rettype function parameters_and_attributes \
+     __attribute__ ((__warning__ (msg)))
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+/* Verify the existence of the function.  */
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+extern rettype function parameters_and_attributes
+# else /* Unsupported.  */
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+_GL_WARN_EXTERN_C int _gl_warn_on_use
+# endif
+#endif
+
+/* _GL_WARN_EXTERN_C declaration;
+   performs the declaration with C linkage.  */
+#ifndef _GL_WARN_EXTERN_C
+# if defined __cplusplus
+#  define _GL_WARN_EXTERN_C extern "C"
+# else
+#  define _GL_WARN_EXTERN_C extern
+# endif
+#endif