Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

src/thread/generic/SDL_syscond.c

@@ -219,7 +219,7 @@ bool SDL_WaitConditionTimeoutNS_generic(SDL_Condition *_cond, SDL_Mutex *mutex,
 }
 
 #ifndef SDL_THREAD_GENERIC_COND_SUFFIX
-SDL_bool SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS)
+bool SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS)
 {
     return SDL_WaitConditionTimeoutNS_generic(cond, mutex, timeoutNS);
 }