compile: use 'compile' script when "-c -o" is used with losing compilers

Do so seen when only source files in the "current" directory are present.

This commit is part of a series of related changes addressing automake
bug#13378 (see also the plan 'PLANS/subdir-objects.txt').

Before this change, Automake-generated C compilation rules mistakenly
passed the "-c -o" options combination unconditionally (even to losing
compiler) when the 'subdir-objects' was used but sources were only
present in the top-level directory.  Issue spotted by Nick Bowler:

  <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13378#35>
  <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13378#44>

We fix this by having Automake redefine AC_PROG_CC to take over the role
of AM_PROG_CC_C_O and to require the 'compile' script unconditionally
(albeit that will continue to be invoked only when inferior compilers
are detected).

Among other things, this means AM_PROG_CC_C_O explicitly is no longer
required; that macro is still supported for backward-compatibility, but
calling it is basically a no-op now.

This change has some pros and some cons (obviously, we believe the former
outweighs the latter).  Here are the most relevant ones:

+ Pros 1:
  Some logic in the Automake script has been simplified.
+ Pros 2:
  That simplification has automatically fixed an actual bug (see
  Nick's mails referenced above; admittedly, that was present only in
  corner-case situations, but still); the test 't/ccnoco4.sh', which
  demonstrated the bug and has been failing so far, now passes.
+ Pros 3:
  Things works more "automagically" now (no need to manually add the
  AM_PROG_CC_C_O macro to configure.ac anymore).

* Cons 1:
  The 'compile' script will be required in all projects using C
  compilation; this will only be a problem for packages not using
  '--add-missing'.  However, such packages are definitely more rare
  than the ones using '--add-missing', and adjusting them will be
  trivial -- just copy the compile script over from the new Automake
  installation.
* Cons 2:
  The copy & paste of autoconf internals hack this change has introduced
  in our "rewrite" of AC_PROG_CC is really an egregious abomination.  It
  can only be justified with the fact that we expect future versions of
  autoconf to implement the semantics we need directly in AC_PROG_CC, so
  that we'll be able to leverage that (since Automake 1.14 will require
  the latest Autoconf version released).

Now, the detailed list of file-by-file changes ...

* automake.in ($seen_cc_c_o): Remove this global variable.
(scan_autoconf_traces): Don't set it, and do not trace the
'AM_PROG_CC_C_O' m4 macro.
(lang_c_rewrite): Remove, no longer needed.
* doc/automake.texi: Adjust expected "autoreconf --install" output
in the amhello example.  Remove statements about the need for the
AM_PROG_CC_C_O macro.  Report it is obsolete now.
* m4/init.m4: Re-write AC_PROG_CC to append checks about whether the
C compiler supports "-c -o" together.  These checks have basically
been ripped out (with adaptations) from the 'AC_PROG_CC_C_O' macro
of Autoconf and ...
* m4/minuso.m4 (AM_PROG_CC_C_O): ... this macro of ours, which has
thus basically become a no-op.
* t/ax/am-test-lib.sh (am_setup_testdir): Also copy the 'compile'
script in the test directory; if we don't do so, every test using
AC_PROG_CC should call automake with the "--add-missing" option, or
copy the 'compile' script itself.
* t/cond11.sh: No need to create a dummy 'compile' script: that is
already brought in by 'am_setup_testdir()', that is automatically
invoked when 'test-lib.sh' is sourced.
* t/add-missing.tap: Adjust: we expect the 'compile' script to be
required by a mere AC_PROG_CC call now.
* t/dist-auxdir-many-subdirs.sh: Likewise.
* t/specflg6.sh: Likewise.
* t/subobj4.sh: Likewise.
* t/cxx-lt-demo.sh: Likewise, and update comments to match.
* t/distcom2.sh: Enhance a little.
* t/dollarvar2.sh: Adjust.
* t/extra-portability.sh: Likewise.
* t/libobj19.sh: Likewise.
* t/per-target-flags.sh: Likewise.
* t/repeated-options.sh: Likewise.
* t/subobj.sh: Likewise, and enhance a little.
* t/ccnoco2.sh: Remove as obsolete.
* t/list-of-tests.mk (handwritten_TESTS): Adjust.
(XFAIL_TESTS): Remove 't/ccnoco4.sh'.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>

1 files changed, 12 insertions, 10 deletions

diff --git a/doc/automake.texi b/doc/automake.texi
index 8ace5e5e0..a333a1c33 100644
--- a/doc/automake.texi
+++ b/doc/automake.texi
@@ -1496,6 +1496,7 @@ command as follows:
 ~/amhello % @kbd{autoreconf --install}
 configure.ac: installing './install-sh'
 configure.ac: installing './missing'
+configure.ac: installing './compile'
 src/Makefile.am: installing './depcomp'
 @end example
 
@@ -3994,10 +3995,9 @@ choose the assembler for you (by default the C compiler) and set
 @item AM_PROG_CC_C_O
 @acindex AM_PROG_CC_C_O
 @acindex AC_PROG_CC_C_O
-This is like @code{AC_PROG_CC_C_O}, but it generates its results in
-the manner required by Automake.  You must use this instead of
-@code{AC_PROG_CC_C_O} when you need this functionality, that is, when
-using per-target flags or subdir-objects with C sources.
+This is an @emph{obsolete wrapper} around @code{AC_PROG_CC_C_O}.
+New code needs not use this macro.  It might be deprecated and
+@emph{retired in future Automake versions}.
 
 @item AM_PROG_LEX
 @acindex AM_PROG_LEX
@@ -4068,6 +4068,13 @@ Invocation, , Using @command{autoupdate} to Modernize
 
 @table @code
 
+@item AM_PROG_CC_C_O
+@acindex AM_PROG_CC_C_O
+@acindex AC_PROG_CC_C_O
+This is an @emph{obsolete wrapper} around @code{AC_PROG_CC_C_O}.  New
+code needs not to use this macro.  It will be deprecated, and then
+removed, in future Automake versions.
+
 @item AM_PROG_MKDIR_P
 @acindex AM_PROG_MKDIR_P
 @cindex @code{mkdir -p}, macro check
@@ -5810,9 +5817,7 @@ different name for the intermediate object files.  Ordinarily a file
 like @file{sample.c} will be compiled to produce @file{sample.o}.
 However, if the program's @code{_CFLAGS} variable is set, then the
 object file will be named, for instance, @file{maude-sample.o}.  (See
-also @ref{Renamed Objects}.)  The use of per-target compilation flags
-with C sources requires that the macro @code{AM_PROG_CC_C_O} be called
-from @file{configure.ac}.
+also @ref{Renamed Objects}).
 
 In compilations with per-target flags, the ordinary @samp{AM_} form of
 the flags variable is @emph{not} automatically included in the
@@ -10245,9 +10250,6 @@ the source file.  For instance, if the source file is
 @file{subdir/file.cxx}, then the output file would be
 @file{subdir/file.o}.
 
-In order to use this option with C sources, you should add
-@code{AM_PROG_CC_C_O} to @file{configure.ac}.
-
 @anchor{tar-formats}
 @item @option{tar-v7}
 @itemx @option{tar-ustar}