The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

https://bugs.musicpd.org/view.php?id=4558

The comment about requiring libtool from 7 years ago is obsolete these
days.

Remove our own songlength database parser.

The latter doesn't exist, and this broke Vorbis-only builds.

Use libvorbis instead of libvorbisfile, which gives us more control
over the decoding process.

Will be needed to compile libxiph.a.

This is a kludge which may break system include path order, see
https://bugs.musicpd.org/view.php?id=4524

The initgroups() manpage says we need to check for _BSD_SOURCE.  The
thing is that glibc deprecated this macro, and doesn't define it
anymore, effectively breaking all MPD supplementary groups.

The real fix is to check for initgroups() availability at configure
time, instead of relying on the deprecated _BSD_SOURCE macro.

We may start using C++ exceptions for error handling soon.

This should not be necessary if libgme was built properly, and it may
break the build with libc++.

Also uses the notification system to display tags.

Not used anymore.