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.

src/pcm_resample_internal.h

@@ -52,7 +52,7 @@ pcm_resample_lsr_32(struct pcm_resample_state *state,
 		    unsigned dest_rate,
 		    size_t *dest_size_r);
 
-#else
+#endif
 
 void
 pcm_resample_fallback_deinit(struct pcm_resample_state *state);
@@ -75,5 +75,3 @@ pcm_resample_fallback_32(struct pcm_resample_state *state,
 			 size_t *dest_size_r);
 
 #endif
-
-#endif