This also reverts the previous commit which was wrong.  When the
Vorbis decoder is disabled, we can't compile VorbisComments.cxx at
all.

Instead of expanding the #ifdef, this commit moves VorbisComments.cxx
to a separate library with dependencies on libvorbis (which was
missing previously, which could also lead to build failures if the
libvorbis headers were in a non-standard directory).

Fixes build failure.

Fixes build failure in StickerCommands.cxx.

More for https://github.com/MusicPlayerDaemon/MPD/issues/42

Prepare for using other crypto libraries, e.g. FFmpeg's libavutil.

Use the "comments" attribute instead of relying on the nullptr
terminator.