The ports `libvorbis`, `readline` and `timidity` would not install on
macOS as a result of using `sed -i` without an extension provided. GNU
sed is available through Homebrew, but it does not replace `sed` by
default.
Instead, provide a new `sed_in_place` function that calls `sed` with the
right arguments.
Also add it as a dependency for the `SDL_mixer` and `SDL2_mixer` ports.
Eawpats GUS patches are installed as part of the port, because without
patches TiMidity++ is not able to generate any sound. The license for
these is "fameware", credit to Andrew Suffield:
https://lists.debian.org/debian-legal/2002/09/msg00137.html
