This does not actually add SDL3 support but adds enough
CMake stuff to make the following succeed:
```bash
cmake -S. -Bbuild-sdl3 -DUSE_SDL3=ON -DDEVILUTIONX_SYSTEM_SDL3=OFF -DDEVILUTIONX_STATIC_SDL3=ON -DNOSOUND=ON
```
`NOSOUND` is needed because `SDL_audiolib` does not support SDL3.