libsndfile: disable external library dependencies

libsndfile: disable external library dependencies

Fixes static linking of pifmrds [1]:

host/usr/bin/arm-linux-gcc -static -o pi_fm_rds rds.o waveforms.o pi_fm_rds.o fm_mpx.o control_pipe.o -lsndfile -lm
.../host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libsndfile.a(flac.o): In function `sf_flac_error_callback':
 flac.c:(.text+0x44c): undefined reference to `FLAC__StreamDecoderErrorStatusString'

host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libsndfile.a(ogg.o): In function `ogg_close':
 ogg.c:(.text+0x10): undefined reference to `ogg_sync_clear'

host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libsndfile.a(ogg_vorbis.o): In function `vorbis_read_sample':
ogg_vorbis.c:(.text+0x26c): undefined reference to `vorbis_synthesis_pcmout'

[1] http://autobuild.buildroot.net/results/9b7/9b7638caa8f3e82e38fb68b0321cb649618a0131

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>