Though these are broken and error prone, as they ignore the possibility
of conflicts between SDL, SDL2 and wrappers thereof discarding useful
include path prefix (SDL2/), since SDL and satellite libraries follow
these it's too hard to stay away.
So don't use prefixes for SDL include paths any longer.
Some SDLMixer tests are muted: Mix_Init(0) doesn't really return mask of
initted sample/music loaders for me, despite the documentation says it
should. SDL_mixer bug?
