configure.ac: Always define __STDC_LIMIT_MACROS.

... which ensures that we get defines like LONG_MAX in C++.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91591
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

configure.ac

@@ -235,7 +235,7 @@ _SAVE_LDFLAGS="$LDFLAGS"
 _SAVE_CPPFLAGS="$CPPFLAGS"
 
 dnl Compiler macros
-DEFINES=""
+DEFINES="-D__STDC_LIMIT_MACROS"
 AC_SUBST([DEFINES])
 case "$host_os" in
 linux*|*-gnu*|gnu*)