Switch to the lowmem tremor branch

in addition to using slightly less memory, this branch also doesn't seem
to have the same issues with `-O2` builds that the main branch has.

lib/tremor/vorbisidec.pc.in

@@ -8,7 +8,7 @@ includedir=@includedir@
 Name: vorbisidec
 Description: vorbisidec is the integer Ogg Vorbis library
 Version: @VERSION@
-Requires.private: ogg
+Requires: ogg
 Conflicts:
-Libs: -L${libdir} -lvorbisidec
+Libs: -L${libdir} -lvorbisidec -lm
 Cflags: -I${includedir}