The ebuild sets REQUIRED_USE="?? ( Xaw3d gtk ) xpm? ( !gtk )". REQUIRED_USE was introduced for the case that preference of one flag over another would break reverse USE dependencies. This is not the case here. Policy reference: <http://devmanual.gentoo.org/general-concepts/use-flags/index.html#conflicting-use-flags> "In order to avoid forcing users to micro-manage flags too much, REQUIRED_USE should be used sparingly. Follow the normal policy whenever it is possible to do a build that will presumably suit the user's needs."