meson: add fixed-point Vorbis (Tremor) decoder support

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

src/decoder/plugins/meson.build

@@ -36,8 +36,9 @@ if flac_dep.found()
   ]
 endif
 
-conf.set('ENABLE_VORBIS_DECODER', libvorbis_dep.found())
-if libvorbis_dep.found()
+conf.set('ENABLE_VORBIS_DECODER', libvorbis_dep.found() or libvorbisidec_dep.found())
+conf.set('HAVE_TREMOR', libvorbisidec_dep.found())
+if libvorbis_dep.found() or libvorbisidec_dep.found()
   decoder_plugins_sources += [
     'VorbisDecoderPlugin.cxx',
     'VorbisDomain.cxx',
@@ -181,6 +182,7 @@ decoder_plugins = static_library(
     libsidplay_dep,
     libsndfile_dep,
     libvorbis_dep,
+    libvorbisidec_dep,
     ogg_dep,
     wavpack_dep,
     wildmidi_dep,