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The comment about requiring libtool from 7 years ago is obsolete these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

Remove our own songlength database parser.

Eliminates duplicate ParseContainerPath() call.

The user manual specifies "encoder", which is consistent with other
output plugins.  "encoding" should be deprecated.

The ScopeExit library uses C++11 initializers, which gcc 4.6 does not
support.  Let's kill support for this ancient incomplete C++11
compiler, nobody should be using it anymore.