move bug #5333 fix to headers

src/atomic/SDL_spinlock.c

@@ -28,12 +28,6 @@
 #include "SDL_mutex.h"
 #include "SDL_timer.h"
 
-#if defined(__WIN32__) && defined(__clang__)
-# ifndef HAVE_GCC_ATOMICS
-# define HAVE_GCC_ATOMICS 1
-# endif
-#endif
-
 #if !defined(HAVE_GCC_ATOMICS) && defined(__SOLARIS__)
 #include <atomic.h>
 #endif