Issue 328: always clean build dir before creating mpkg dist.
Issue 328: always clean build dir before creating mpkg dist.

niklaskorz/pyglet,niklaskorz/pyglet,seeminglee/pyglet64,adamlwgriffiths/Pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,seeminglee/pyglet64,adamlwgriffiths/Pyglet