Use _Exit() when available

src/SDL_assert.c

@@ -135,9 +135,13 @@ static void SDL_ExitProcess(int exitcode)
     emscripten_cancel_main_loop();  /* this should "kill" the app. */
     emscripten_force_exit(exitcode);  /* this should "kill" the app. */
     exit(exitcode);
+#else
+#ifdef HAVE__EXIT /* Upper case _Exit() */
+    _Exit(exitcode);
 #else
     _exit(exitcode);
 #endif
+#endif
 }