Define _GNU_SOURCE when building SDL

src/thread/pthread/SDL_sysmutex.c

@@ -20,9 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
 #include <errno.h>
 #include <pthread.h>
 

src/thread/pthread/SDL_syssem.c

@@ -20,9 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
 #include <errno.h>
 #include <pthread.h>
 #include <semaphore.h>