cmake HAVE_JUST_SDL_H examples

Ravbug/UnityHubNative wxWidgets/3rdparty/libwebp/CMakeLists.txt :675

check_c_source_compiles(
      "
        #define SDL_MAIN_HANDLED
        #include \"SDL.h\"
        int main(void) {
          return 0;
        }
      "
      HAVE_JUST_SDL_H)