If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

Move from PcmClamp().

Support for conversion from float to 16, 24 and 32 bit integer
samples.

Support 32 bit samples with software mixer.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

All what's left in pcm_utils.h is the pcm_range() utility function,
which is only used internally by pcm_volume and pcm_mix.

Moved the software mixing code (used by crossfading) to a separate
library.

Moved the software volume code to a separate library.

We are going to split the pcm_utils.c library, and pcm_range() will be
useful for several sub libraries.

It may be desirable to change the range of integer volume levels
(e.g. to 1024, which may utilize shifts instead of expensive integer
divisions).  Introduce the constant PCM_VOLUME_1 which describes the
integer value for "100% volume".  This is currently 1000.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

Separate the resampling code from the rest of pcm_utils.c.  Create two
sub-libraries: pcm_resample_libsamplerate.c and
pcm_resample_fallback.c.

Copied and adapted code from the mp3 decoder plugin.  This library now
replaces the old and low-quality function pcm_convert_24_to_16().