Follow lead in thunar-0.5.0_rc2-r2 ebuild and change USE="xslt" to USE="doc" because that is what it does; builds documentation.
Then delete use.local.desc entries: xfce-base/thunar:xslt - Build HTML based docs from xml. xfce-extra/terminal:xslt - Build HTML based docs from xml.
FIXED! KTHXBYE! :)