This makes ccache more efficient when recompiling with different
plugins.

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).

Fixes build failure.

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

Use the "comments" attribute instead of relying on the nullptr
terminator.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Move code from the Vorbis and FLAC decoder plugins.

To replace g_strdup().

Replaces GLib's g_ascii_strncasecmp().