libopus and libspeex can not be statically linked in same application
trying to link libopus and libspeex in a static application on OS X (i.e. ffmpeg) fails due to duplicate symbols:
ld: duplicate symbol _pitch_xcorr in lib/libopus.a(pitch.o) and lib/libspeex.a(ltp.o)
to work around this, pitch_xcorr in opus could be renamed to opus_pitch_xcorr