build: don't rely on implicit GNU extensions

Currently libwayland assumes GNU extensions will be available, but
doesn't define the C standard to use. Instead, let's unconditionally
enable POSIX extensions, and enable GNU extensions on a case-by-case
basis as needed.

Signed-off-by: Simon Ser <contact@emersion.fr>

cursor/xcursor.c

@@ -23,6 +23,7 @@
  * SOFTWARE.
  */
 
+#define _GNU_SOURCE
 #include "xcursor.h"
 #include <stdio.h>
 #include <stdlib.h>