Use AC_USE_SYSTEM_EXTENSIONS to get _GNU_SOURCE, _XOPEN_SOURCE etc.

Similar to Bug #684123 in GLib.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen

gdk/wayland/gdkdevice-wayland.c

@@ -15,7 +15,6 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>