Meanwhile, libkface has an "own" source code release, independent of Digikam. Currently, you can find the package at http://download.kde.org/unstable/applications/14.11.97/src/libkface-14.11.97.tar.xz According to Gilles Caulier from Digikam, they will continue to include the libkface sources in the Digikam releases for now, but they want to encourage people to use the new dedicated release instead. So probably, you want to add ebuilds using this release now.
commit 50e514fcb4c915d75c63ffb9758c06ed8e0b163d Author: Manuel Rüger <mrueg@gentoo.org> Date: Thu Dec 18 00:11:54 2014 +0100 [kde-apps/libkface] Add 14.12.0 release. Clean headers. Package-Manager: portage-2.2.15
