fix cute_sound warnings on MacOS

[johnpayne-dev]

Fixes the lines that do nothing and give a warning.

I also added #ifndef SDL_h_ around the SDL2 include because it's possible for the user to want to use their own SDL2 include, for example in my case I build SDL2 from source so I include by doing #include "SDL.h".

[RandyGaul]

Looks good, thanks!