As a side effect, "playlistclear" no longer creates a new playlist if
the given one doesn't yet exist.

This is based on a patch from Ian Scott in 2014. It was never committed,
so I figured I'd fix the outstanding issue and resubmit it.

https://www.mail-archive.com/mpd-devel%40musicpd.org/msg00139.html

Closes https://bugs.musicpd.org/view.php?id=4537 .

Removed the 'cancel' function because it violates 'single producer,
single consumer'.

release v0.19.18

Fixes memory leak after stream failure.  See
https://bugs.musicpd.org/view.php?id=4562

Required for Boost 1.61, which uses that overload in a BOOST_ASSERT().

Turns out the libavcodec version numbers are not linear;  the feature
was added in FFmpeg 3.1 commit 998e1b8, libavcodec 57.14.0; but FFmpeg
3.0 has version 57.48.101.  Ouch!

https://bugs.musicpd.org/view.php?id=4560

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

Was accidently added by commit cafc266e

The ReSIDBuilder constructor cannot fail.

The comment about requiring libtool from 7 years ago is obsolete these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

Eliminate duplicate SidTune construction.