Trying to emerge app-text/docbook-xml-dtd everything goes succesfully except following message: /var/tmp/portage/app-text/docbook-xml-dtd-4.5-r1/temp/environment: line 2227: build-docbook-catalog: command not found Seems that it wants build-docbook-catalog to be installed, so I think it should be in DEPEND block of all versions of app-text/docbook-xml-dtd (I've tried 4.2-r2 and 4.5-r1).
It seems this dependency has been added already.