Port _AC_C_C99_TEST_MAIN to -Wc++-compat

Problem reported by Vincent Lefevre in:
https://lists.gnu.org/r/autoconf-patches/2025-08/msg00004.html
* lib/autoconf/c.m4 (_AC_C_C99_TEST_MAIN):
Don’t use malloc.

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index a36f56c7e8fd5b358c6f531d16bfb50043aee001..689e506d6f069fe1fd060fea3bc4bc09085cf3ee 100644 (file)
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1324,8 +1324,8 @@ ac_c_conftest_c99_main='
   test_varargs_macros ();
 
   // Check flexible array members.
-  struct incomplete_array *ia =
-    malloc (sizeof (struct incomplete_array) + (sizeof (double) * 10));
+  static struct incomplete_array *volatile incomplete_array_pointer;
+  struct incomplete_array *ia = incomplete_array_pointer;
   ia->datasize = 10;
   for (int i = 0; i < ia->datasize; ++i)
     ia->data[i] = i * 1.234;