While setting up a new system with FreeBSD 9.0, the port of the awesome window manager would not build. More specifically, generating manpages failed with:

xmlto: /usr/ports/x11-wm/awesome/work/awesome-3.4.10/manpages/man1/awesome.1.xml does not validate (status 3)

After some searching, I discovered the solution: Simply install the port textproc/docbook-xml-450 in addition to the dependency textproc/docbook-450 of the awesome window manager.