[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]

Now that the remaining known bugs in poll() implementation are fixed,
we can go on without the GLib implementation.

If we have libyajl 2.0.1 (without a pkg-config file), our configure.ac
would assume this is the libyajl 1.0 API, because the function
yajl_alloc() exists in both.  This commit changes the library check to
the function yajl_parse_complete() which was removed in the 2.0 API.
This fixes build failure with libyajl 2.0.1.

Due to recent protocol additions ("addtagid", "cleartagid").

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Rename HAVE_WINDOWS to host_is_windows for consistency.
Use 'yes' as true value instead of '1' for this variable.
Use test on this variable instead of case where applicable.

Remove compatibility code.

This change adds two configuration options:

  --with-eventloop=[glib|internal|auto]
  --with-pollmethod=[epoll|auto]

First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.

The option "--enable-fluidsynth" was documented to be "auto" by
default, when it was really "no".

bzip2-compressed tarballs are obsolete, xz is better and widely
understood.

Prior to version 0.3, the "length" callback returned a "long" instead
of AFfileoffset.  Now that this API bug fix is a few years old, let's
drop 0.2 support for good.

Without -Werror, clang will not fail on -ftree-vectorize.

Allows advanced compiler optimization.

Most importantly: always disable C++ exceptions.  It was surprising to
see MPD terminate due to an unexpected exception.

There hasn't been a maintainer for this plugin for years.  It may even
fail to build.

Fixes the OpenAL detection which was broken because enable_osx was
uesd which was set later.