Define _GNU_SOURCE when building SDL

src/stdlib/SDL_string.c

@@ -18,15 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
-
-#if defined(__clang_analyzer__) && !defined(SDL_DISABLE_ANALYZE_MACROS)
+#if defined(__clang_analyzer__)
 #define SDL_DISABLE_ANALYZE_MACROS 1
 #endif
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE 1
-#endif
-
 #include "../SDL_internal.h"
 
 /* This file contains portable string manipulation functions for SDL */