xfce4-session-4.11.0-r1 doesn't build with USE=-udev and missing upower packages. It fails on: checking for upower-glib >= 0.9.8... not found *** The required package upower-glib was not found on your system. Reproducible: Always
http://bugzilla.xfce.org/show_bug.cgi?id=9952#c17 Fixed in Portage with a temporary workaround that applies the patch only with USE="udev".