All plattform stuff is now in Standard.h, while C4Include.h just includes
a bunch of headers. Mostly headers from the standard library, because
those change very seldom.
Also remove the ptrdiff_t typedef, properly include <cstddef> instead.
USE_FMOD define renamed to HAVE_FMOD to bring it in line with HAVE_SDL_MIXER.
MCI removed since nobody was using it anyway, and half of its functions were
stubs.