Define _GNU_SOURCE when building SDL

src/SDL_internal.h

@@ -21,6 +21,15 @@
 #ifndef _SDL_internal_h
 #define _SDL_internal_h
 
+/* Many of SDL's features require _GNU_SOURCE on various platforms
+   You can turn this off if necessary by using -D_GNU_SOURCE=0
+ */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#elif !_GNU_SOURCE
+#undef _GNU_SOURCE
+#endif
+
 #include "dynapi/SDL_dynapi.h"
 
 #if SDL_DYNAMIC_API
@@ -33,6 +42,6 @@
 
 #include "SDL_config.h"
 
-#endif
+#endif /* _SDL_internal_h */
 
 /* vi: set ts=4 sw=4 expandtab: */