pcm_resample: choose the fallback resampler at runtime

Even if libsamplerate support is enabled, compile the fallback
resampler.  When the user specifies the option
"samplerate_converter=internal", it is chosen in favor of
libsamplerate.  This may help users with a weak FPU who don't want to
compile a custom MPD from source, because the fallback resampler does
not use floating point operations.

doc/mpd.conf.5

@@ -158,6 +158,11 @@ distortions.
 Linear Interpolator (4)
 
 Linear interpolator, very fast, poor quality.
+.TP
+internal
+
+Poor quality, no floating point operations.  This is the default (and
+only choice) if MPD was compiled without libsamplerate.
 .RE
 .IP
 For an up-to-date list of available converters, please see the libsamplerate