[Suggestion] Enable Fluidsynth support for sdl2_mixer

Right now, SDL2_mixer that is available by default on Alpine lacks any support for MIDI playback. This, however, can be fixed, if it is built with FluidSynth support. (TiMidity could be an option, but it is not packaged in alpine at the moment.)

Adding an --enable-music-midi-fluidsynth flag to the package's ./configure command and listing a dependency on fluidsynth should be enough for it to work, and after testing the modification locally, it seems to work, allowing Chocolate Doom to use FluidSynth's capabilities to play back MIDIs.