Fixes a build failure.

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).

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

And disable libvorbis detection if Tremor was explicitly enabled.

This fixes a crash bug caused by libvorbis/Tremor ABI conflict caused
by commit 4f7d52db

Re-add build support for the fixed-point Vorbis (Tremor) decoder, which
was dropped when switching from Autotools to Meson.

Note, that it is not possible to build both, the Vorbis and the Tremor
decoder.

Closes: #405

Without the Vorbis decoder, `libvorbis` is never detected, leading to
linker failures when attempting to build the Vorbis encoder.

Fixes linker failure when building without FLAC support.

Closes #400

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!