configure: require pkg-config for libvorbis

libvorbis comes with pkg-config files since at least v1.0.1, way back in 2003.

We need the two checks for vorbis and vorbisenc because we use functions from
both and Xiph considers them separate libraries.

The check is inverted (vorbis first then vorbisenc) because add_extralibs()
prepends to EXTRALIBS instead of appending. For both shared and static linking
the order didn't seem to matter anyway, testing with MinGW.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
