configure.ac: Add AC_GNU_SOURCE

We define it here and there already, and if we are to include private
cogl files, we need it in more places. Lets just add it everywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=768976

src/core/main.c

@@ -41,7 +41,6 @@
  * to investigate, read main(), meta_display_open(), and event_callback().
  */
 
-#define _GNU_SOURCE
 #define _XOPEN_SOURCE /* for putenv() and some signal-related functions */
 
 #include <config.h>