Add build for extra library dependencies.

Replace the -libsamplerate parameter with -extra-libs, which builds
a number of extra libraries that Chocolate Doom can now make use of:

  libpng
      - so we can save PNG format screenshots
  libogg / libvorbis / libflac
      - for SDL_mixer, so we can play high quality substitute music
        recordings.

Download all these dependencies from chocolate-doom.org so that we
have a reliable central place that they can be obtained; fetching
them from individual websites is bound to just end in tears.