If the shout plugin is disabled, shout_plugin.c isn't compiled at all,
no need to check the macro definition.

I have found something that looks like a bug in MPD:
- When a song is finished, the next one is played and the 'player'
  event is emitted.
- When the client sends the status command just after this event, the
  songid is the new one but the 'elapsed' time is not reseted to 0.

This is problem because I have implemented the solution using a timer
on client side to compute the elapsed time but with this bug the
elapsed time continues to be incremented on a new song.

When ao_open_live() failed, MPD would ignore the error code in
"errno".  Make it print a meaningful error message.

The function audioOutputAo_error() did not implement all possible
libao error codes.  Support the rest of them, and fall back to
strerror().

SEEK_SET is defined by unistd.h.  Explicitly include it.

The music pipe audio format bugs seem to be fixed (hopefully).
Disable music_pipe_check_format() for now, since it consumes a lot of
CPU cycles.

Renamed the local variable "basename", which shadows the POSIX
function basename().

We shouldn't pass strerror() where a format string is expected.

assert_static() will help us to find false asserts in compile time. Of
course it only works in case of expressions which can be evaluated
compile time. It cannot be used in global scope.

The client may provide the names of idle events as arguments to the
"idle" command to inform MPD that it is only interested in these
events.

Refuse to play audio formats which are not supported by MPD.

Pushing buffers with a zero length into the MPD core causes failures;
don't let them pass beyond the decoder plugi API.

Removed the superfluous my_usleep() call.

When MPD quits in a non-clean way, the state file isn't written, and
on the next start, MPD time warps to the previous clean shutdown.
Save the state file every 5 minutes; this will probably be
configurable at a later time.

Note that we don't set a wakeup timer for that: when there is no MPD
traffic, MPD won't wake up to save the state file.  This minor bug is
tolerated, because usually there is no change in MPD's state when the
main thread is idle.

If the caller attempts to seek only a few bytes forward, chances are
good that the offset is already in the buffer.  In this case, simply
fast-forward the buffer.

Added some assertions.