The rebuilding of doc which requires the docbook infrastructures does not have a user flag in order to disable it.
Created attachment 136123 [details, diff] dia 0.96.1 ebuild patch for db2html use flag support
Created attachment 136125 [details] db2html local use flag definition
commited to CVS. Note that the doc use flag is appropriate for this use. It's mainly used to rebuild documentation and fix the cross-references on gnome related applications.