Define SDL_PLATFORM_* macros instead of underscored ones (#8875)

cmake/sdlchecks.cmake

@@ -849,7 +849,7 @@ macro(CheckPTHREAD)
         check_c_source_compiles("
             #include <pthread.h>
             int main(int argc, char **argv) {
-              #ifdef __APPLE__
+              #ifdef SDL_PLATFORM_APPLE
               pthread_setname_np(\"\");
               #else
               pthread_setname_np(pthread_self(),\"\");