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,

src/lib/xiph/VorbisComments.cxx

@@ -29,8 +29,6 @@
 #include "util/StringView.hxx"
 #include "config.h"
 
-#ifdef ENABLE_VORBIS_DECODER
-
 #ifndef HAVE_TREMOR
 #include <vorbis/codec.h>
 #else
@@ -65,8 +63,6 @@ VorbisCommentToReplayGain(ReplayGainInfo &rgi,
 	return found;
 }
 
-#endif /* ENABLE_VORBIS_DECODER */
-
 static void
 vorbis_scan_comment(StringView comment, TagHandler &handler) noexcept
 {

src/lib/xiph/meson.build

@@ -40,6 +40,7 @@ endif
 if not libogg_dep.found() and not libflac_dep.found()
   xiph_dep = dependency('', required: false)
   ogg_dep = dependency('', required: false)
+  vorbis_dep = dependency('', required: false)
   flac_dep = dependency('', required: false)
   subdir_done()
 endif
@@ -82,6 +83,29 @@ else
   ogg_dep = dependency('', required: false)
 endif
 
+if libvorbis_dep.found() or libvorbisidec_dep.found()
+  vorbis = static_library(
+    'vorbis',
+    'VorbisComments.cxx',
+    include_directories: inc,
+    dependencies: [
+      libvorbis_dep,
+      libvorbisidec_dep,
+    ],
+  )
+
+  vorbis_dep = declare_dependency(
+    link_with: vorbis,
+    dependencies: [
+      ogg_dep,
+      libvorbis_dep,
+      libvorbisidec_dep,
+    ],
+  )
+else
+  vorbis_dep = dependency('', required: false)
+endif
+
 if libflac_dep.found()
   flac = static_library(
     'flac',