lib/xiph/VorbisComments: move to separate static library

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).

src/decoder/plugins/meson.build

@@ -36,9 +36,9 @@ if flac_dep.found()
   ]
 endif
 
-conf.set('ENABLE_VORBIS_DECODER', libvorbis_dep.found() or libvorbisidec_dep.found())
+conf.set('ENABLE_VORBIS_DECODER', vorbis_dep.found())
 conf.set('HAVE_TREMOR', libvorbisidec_dep.found())
-if libvorbis_dep.found() or libvorbisidec_dep.found()
+if vorbis_dep.found()
   decoder_plugins_sources += [
     'VorbisDecoderPlugin.cxx',
     'VorbisDomain.cxx',
@@ -181,8 +181,7 @@ decoder_plugins = static_library(
     libopus_dep,
     libsidplay_dep,
     libsndfile_dep,
-    libvorbis_dep,
-    libvorbisidec_dep,
+    vorbis_dep,
     ogg_dep,
     wavpack_dep,
     wildmidi_dep,