xml2-config is dead, long live pkg-config
The Debian unstable builds are failing with:
checking for libxml2 library... configure: error: xml2-config returns badness cd build && tail -v -n \+0 config.log
and while in fact it's an error, the
xml2-config is going to be obsoleted soon, and we should try using
pkg-config to find libxml2 first.