Fixes build failure on OS X, closes #44.  With the other plugins,
that's not critical, because those use the AudioOutputWrapper, which
hides this problem.

See commit 71f0ed8b

See commit 788e3b31 for an explanation.

release v0.20.7

Closes #34

Closes #43

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Closes #31

Fixes a potential crash bug which is actually unreachable, because the
"pcm" plugin is never invoked when there is no (matching) MIME type.