Removed GLib logging wrappers which are not used anymore.

If a log message does not include a newline character, append it.

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

gmtime_r() is not available.

These methods are unused.

Fix for bug #1491.

I tried to search for a certain composer in my collection, but only
non-mp4 files showed up. The source code reveals that this tag is not
read. This can be fixed by reading the 'Writer' tag field, in
mp4_plugin.c, in function mp4_load_tag.

I actually tried this, and after compiling with those lines added,
also mp4 (.m4a) files showed up when searching for a composer.

All archive plugins used to overwrite $MPD_LIBS, ignoring the existing
value.  Make them append instead, keeping the old value.

Don't bother to look for libbz2, libzzip and libiso9660 if the plugins
are disabled.

We're now using GLib threading.

Replaced myFgets() with fgets() + g_strchomp().