Added native atomic operations for Windows, Mac OS X, and gcc compiler intrinsics.

Changed the CAS return value to bool, so it's efficient with OSAtomicCompareAndSwap32Barrier()
Added an atomic test adapted from code by Michael Davidsaver

include/SDL_config_iphoneos.h

@@ -43,6 +43,8 @@ typedef unsigned long uintptr_t;
 
 #define SDL_HAS_64BIT_TYPE	1
 
+#define HAVE_GCC_ATOMICS	1
+
 #define HAVE_ALLOCA_H		1
 #define HAVE_SYS_TYPES_H	1
 #define HAVE_STDIO_H	1