disabling the discovery of SDL13+ on desktop. SDL13 has become SDL2 with a completely different ABI and will require a new FindSDL2 module for Cmake to be found; for current sdl development installations, hedgewars will either use the compatibility layer (present in sdl1.3 but not in sdl2) or just fail to build (in case sdl2 is installed but sdl1.2.* is not). whew
