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!