We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

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

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.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

See code comment.  Bug was reported against MPD, but it's really an
alsa-lib bug.

 https://bugs.musicpd.org/view.php?id=4662

Some missing bits when converted to C++ exceptions...

Fixes a potential deadlock introduced by commit 94528735

Using variable-size objects is not worth the trouble here.  Let's drop
this and use existing and simpler code.

Fixes another buffer overflow: if the stream has a very long title or
URL, resulting in a metadata string of more than 2 kB, icy_string[0]
is a negative value, which gets casted to size_t - ouch!

 https://bugs.musicpd.org/view.php?id=4652

Fixes a buffer overflow due to the bad formula rounding the buffer
size up.  At the same time, remove the "+1" from the meta_length
calculation, which takes the padding into account and at the same time
implements proper rounding.

Why did this cast exist??

During UnlockActivate() while the mutex is unlocked, the IOThread can
set a new error condition, and will never again wake up the
OutputThread.  This race condition can cause a deadlock in the
OutputThread.

.. and not the input data.

Regression from commit b1c7649e (integer underflow).

 https://bugs.musicpd.org/view.php?id=4639

Use SND_PCM_NONBLOCK, and perform all snd_pcm_writei() calls in the
IOThread.  Use a lockless queue to copy data from the OutputThread to
the IOThread.

This rather major change aims to improve MPD's internal latency.  All
waits are now under MPD's control, instead of blocking inside
libasound2.

As a side effect, an output's filter is now decoupled from the actual
device I/O, which solves a major latency problem with the conversion
filter on slow CPUs and small period buffers.  See:

 https://bugs.musicpd.org/view.php?id=3900

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

Eliminate dependency on io_thread_get().