For Darwin, code expects SDL_mixer to be in the framework path.
Use that here too.
Add search SDL_mixer framework paths for headers instead of using <Framework/Header.h> notation
Merge commit 'osx-fixes' into unification/master

SDL on OS X does not include pkg-config files, so assume the user put it
in the OS X specific "framework" directory.

Reported by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/14
Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/15
Search for SDL_mixer pkg-config too

Users might install SDL and SDL_mixer to separate paths, so search for
SDL_mixer.pc too.