Import SDL2_mixer-2.0.0 as wip/SDL2_mixer.

SDL_mixer is a sample multi-channel audio mixer library. It supports any number
of simultaneously playing channels of 16 bit stereo audio, plus a single channel
of music, mixed by the popular FLAC, MikMod MOD, Timidity MIDI, Ogg Vorbis, and
SMPEG MP3 libraries.

As of SDL_mixer 1.2.7, FLAC, MikMod, Ogg Vorbis and MP3 loading libraries are
dynamically loaded, so if you don't need to load those formats, you don't need
to include those shared libraries.
+SDL2_mixer
Remove GNU_CONFIGURE_LIBDIR.

Rationale: undocumented, easily replaced with "CONFIGURE_ARGS+=--libdir="
and only used by 10 packages.