build-sys: Deprecate libsamplerate support

output DEPRECATED warnings for libsamplerate in configure and
PA daemon's log

libsamplerate offers no particular advantage over the speex
resampler and is distributed under GPL; support for it will be removed
in one of the next releases

v2: (thanks Arun Raghavan)
* log a warning (instead of info)

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>