vorbis.pc.in assumes -lm, this should be detected first and then added if needed

Not all platforms require -lm, so instead of assuming -lm is needed, a check should be done and then add it in if needed. Perhaps change vorbis.pc.in to use ${libm}, default it to "" and change it to "-lm" There is an AC_CHECK_LIB (m, cos, VORBIS_LIBS="-lm", VORBIS_LIBS="") in confirgure.ac, so perhaps it's as simple as changing the -lm in vorbis.pc.in to ${vobis_libs} or something like that.