conftools: Fix regression with detection of -Wstrict-prototypes

The issue was that the code generated by AC_LANG_PROGRAM
produces warnings, and hence -Werror makes detection fail:

$ gcc -x c -Werror -Wstrict-prototypes - <<<'int main() {}'; echo $?
<stdin>:1:5: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
cc1: all warnings being treated as errors
1

expat/conftools/ax-check-compile-flag.m4

@@ -9,7 +9,7 @@
 # DESCRIPTION
 #
 #   Check whether the given FLAG plus -Werror works with the current
-#   language's compiler or gives an error.
+#   language's compiler — C or C++ — or gives an error.
 #
 #   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
 #   success/failure.
@@ -56,7 +56,7 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 5
+#serial 6
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 [AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
@@ -64,7 +64,9 @@ AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
 AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
   _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 -Werror $1"
-  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+  AC_COMPILE_IFELSE([m4_default([$5],
+                                [AC_LANG_SOURCE(
+                                   [[int main(void) { return 0; }]])])],
     [AS_VAR_SET(CACHEVAR,[yes])],
     [AS_VAR_SET(CACHEVAR,[no])])
   _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])