Configure failed to find vorbis & vorbisenc on Linux with the errors:
ERROR: vorbis not found using pkg-config
ERROR: vorbisenc not found using pkg-config
the issue was caused by configures test program failing to link the test
code against the static library because functions in libm weren't found.
Resolve by adding `$libm_extralibs` to libvorbis.
Note that it's strange this issue didn't come up before,
since it occurs on a clean build it seems necessary.
b27ff093ca