This is required build-time only, and is automagically set by the eclass when KDE_HANDBOOK is enabled. Of course this will be an issue for users with handbook turned off. So, we either need some eclass function to punt the dependency (which would be useful for the QtTest issue we are having in other packages too), or just decide to make handbooks mandatory (I wonder why they are optional originally, just because or is there a big deptree?)