configure.ac: find more libraries with pkg-config

Detect the following libraries with pkg-config: libshout, libid3tag,
libmad.

src/Makefile.am

@@ -196,14 +196,20 @@ mpd_SOURCES = \
 mpd_CFLAGS = $(MPD_CFLAGS)
 mpd_CPPFLAGS = \
 	$(AO_CFLAGS) $(ALSA_CFLAGS) \
+	$(SHOUT_CFLAGS) \
 	$(OGGVORBIS_CFLAGS) $(VORBISENC_CFLAGS) \
 	$(patsubst -I%/FLAC,-I%,$(FLAC_CFLAGS)) \
 	$(AUDIOFILE_CFLAGS) $(LIBMIKMOD_CFLAGS) \
+	$(ID3TAG_CFLAGS) \
+	$(MAD_CFLAGS) \
 	$(GLIB_CFLAGS)
 mpd_LDADD = $(MPD_LIBS) $(MP4FF_LIB) \
 	$(AO_LIBS) $(ALSA_LIBS) \
+	$(SHOUT_LIBS) \
 	$(OGGVORBIS_LIBS) $(VORBISENC_LIBS) $(FLAC_LIBS) \
 	$(AUDIOFILE_LIBS) $(LIBMIKMOD_LIBS) \
+	$(ID3TAG_LIBS) \
+	$(MAD_LIBS) \
 	$(GLIB_LIBS)
 
 DIST_SUBDIRS = mp4ff