Use stdbool for SDL_bool

This helps the compiler warn people when they're doing something like "if (SDL_Init(0) < 0)"

src/SDL_internal.h

@@ -121,16 +121,6 @@
 #include <float.h>
 #endif
 
-#ifndef __cplusplus
-#ifdef HAVE_STDBOOL_H
-#include <stdbool.h>
-#else
-typedef int bool;
-#define true    1
-#define false   0
-#endif
-#endif // !__cplusplus
-
 // If you run into a warning that O_CLOEXEC is redefined, update the SDL configuration header for your platform to add HAVE_O_CLOEXEC
 #ifndef HAVE_O_CLOEXEC
 #define O_CLOEXEC 0