remove warning and fix small bug

removed _GNU_SOURCE, which is set globally in mpd

Added SND_CTL_TLV_DB_GAIN_MUTE which fixes rounding issue on IQaudio
see http://www.alsa-project.org/alsa-doc/alsa-lib/group___control.html

src/mixer/plugins/volume_mapping.cxx

@@ -31,8 +31,6 @@
  */
 
 #define _ISOC99_SOURCE /* lrint() */
-#define _GNU_SOURCE /* exp10() */
-//#include "aconfig.h"
 #include <math.h>
 #include <stdbool.h>
 #include "volume_mapping.hxx"
@@ -43,6 +41,7 @@
 #endif /* __UCLIBC__ */
 
 #define MAX_LINEAR_DB_SCALE	24
+#define SND_CTL_TLV_DB_GAIN_MUTE   -9999999
 
 static inline bool use_linear_dB_scale(long dBmin, long dBmax)
 {