configure: Add explicit checks for all GNU99 extensions used.

Some compilers (clang) claim to support -std=gnu99 but don't actually
implement all extensions we use in the code. Producing really hard to
parse errors. Add explicit checks for some of the other language
extensions we use, Nested Functions and Arrays of Variable Length,
to the configure check to catch such issues early.

https://bugzilla.redhat.com/show_bug.cgi?id=1211357

Signed-off-by: Mark Wielaard <mjw@redhat.com>

1 files changed, 26 insertions, 3 deletions

diff --git a/configure.ac b/configure.ac
index c4b818d0..ed2c9644 100644
--- a/configure.ac
+++ b/configure.ac
@@ -79,15 +79,38 @@ m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 AC_CHECK_TOOL([READELF], [readelf])
 AC_CHECK_TOOL([NM], [nm])
 
-AC_CACHE_CHECK([for gcc with C99 support], ac_cv_c99, [dnl
+# We use -std=gnu99 but have explicit checks for some language constructs
+# and GNU extensions since some compilers claim GNU99 support, but don't
+# really support all language extensions. In particular we need
+# Mixed Declarations and Code
+# https://gcc.gnu.org/onlinedocs/gcc/Mixed-Declarations.html
+# Nested Functions
+# https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html
+# Arrays of Variable Length
+# https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
+AC_CACHE_CHECK([for gcc with GNU99 support], ac_cv_c99, [dnl
 old_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -std=gnu99"
 AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
-int foo (int a) { for (int i = 0; i < a; ++i) if (i % 4) break; int s = a; return s;}])],
+int foo (int a)
+{
+  for (int i = 0; i < a; ++i) if (i % 4) break; int s = a; return s;
+}
+
+double bar (double a, double b)
+{
+  double square (double z) { return z * z; }
+  return square (a) + square (b);
+}
+
+void baz (int n)
+{
+  struct S { int x[[n]]; };
+}])],
 		  ac_cv_c99=yes, ac_cv_c99=no)
 CFLAGS="$old_CFLAGS"])
 AS_IF([test "x$ac_cv_c99" != xyes],
-      AC_MSG_ERROR([gcc with C99 support required]))
+      AC_MSG_ERROR([gcc with GNU99 support required]))
 
 AC_CACHE_CHECK([for __thread support], ac_cv_tls, [dnl
 # Use the same flags that we use for our DSOs, so the test is representative.