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;

test/testrwlock.c

@@ -31,7 +31,7 @@ static SDLTest_CommonState *state;
 static void DoWork(const int workticks)  /* "Work" */
 {
     const SDL_ThreadID tid = SDL_GetCurrentThreadID();
-    const SDL_bool is_reader = tid != mainthread;
+    const bool is_reader = tid != mainthread;
     const char *typestr = is_reader ? "Reader" : "Writer";
 
     SDL_Log("%s Thread %" SDL_PRIu64 ": ready to work\n", typestr, tid);