Updated Makefile and relevant C files to allow 2 flags during make
to specify:
  - whether using libao or libasound: make USE=ao/alsa, and
  - whether to use direct ALSA mixer (only in libasound) to
    control volume or use softare calculated volume -
    make USE_ALSA_VOLUME=1

Hence, to compile for ALSA and use ALSA volume control, compile
with:
    make USE=alsa USE_ALSA_VOLUME=1

To compile with libao, compile with:
    make USE=ao USE_ALSA_VOLUME=0

Something like that