Reinstate _GNU_SOURCE

This was removed at some point creating a difference between downstream and
upstream. The reason for doing so is unclear, so I'm reinstating it in the
interest of minimizing the downstream diff.

If it is needed, it should be applied as a patch from scripts/grab-upstream.sh.

scripts/grab-upstream.sh

@@ -51,6 +51,9 @@ if test -n "$(git status --porcelain)"; then
     echo "  2. Run scripts/grab-upstream.sh from the top-level directory"
     echo "  3. Submit the result as a pull request"
     echo
+    echo "If you must apply patches, please do it in grab-upstream.sh so that"
+    echo "these patches are described programmatically."
+    echo
   } 1>&2
   exit 1
 fi

v3.2/glfw/glfw/src/wl_window.c

@@ -24,6 +24,8 @@
 //
 //========================================================================
 
+#define _GNU_SOURCE
+
 #include "internal.h"
 
 #include <stdio.h>