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/pthread/SDL_syscond.c

@@ -78,7 +78,7 @@ void SDL_BroadcastCondition(SDL_Condition *cond)
     pthread_cond_broadcast(&cond->cond);
 }
 
-SDL_bool SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS)
+bool SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS)
 {
 #ifndef HAVE_CLOCK_GETTIME
     struct timeval delta;