Highlights:
*Building with msys2 can now take advantage of sdl-config and sdl2-config if present.
*Add a header check to error out if SDL_mixer's major version does not match SDL.
*Building with frameworks on OS X works again.
Notes:
*Those cross compiling, particularly targeting Windows, may need to invoke make with SDLCONFIG='' to avoid bringing in libs and headers from the host.
git-svn-id: https://svn.eduke32.com/eduke32@5778 1a8010ca-5511-0410-912e-c29ae57300e0
