Import SDL_mixer VS2010 project file.
Adjust source and output paths.
Disable MIDI and FLAC.

These should be uncommon enough we can omit them and save a little
space.
Disable MP3 support.

Some Linux distros do not include MP3-related packages in their core
repos for licensing reasons, so we'll just use Ogg Vorbis instead.
Fix output file suffix and include paths.
Apply disabled settings to other configs.
Add SDL2_mixer project.
Add build instructions.