Use C99 bool internally in SDL

src/loadso/dlopen/SDL_sysloadso.c

@@ -57,7 +57,7 @@ SDL_FunctionPointer SDL_LoadFunction(void *handle, const char *name)
     void *symbol = dlsym(handle, name);
     if (!symbol) {
         // prepend an underscore for platforms that need that.
-        SDL_bool isstack;
+        bool isstack;
         size_t len = SDL_strlen(name) + 1;
         char *_name = SDL_small_alloc(char, len + 1, &isstack);
         _name[0] = '_';