=x11-misc/lightdm-1.19.0 compiles and runs fine on PowerPC 64 architecture. Tested with an Imac G5 machine. I suggest to add ~ppc64 keyword following ppc one, as the two are the same architecture.
* x11-misc/lightdm [gentoo] Maintainer: hwoarang@gentoo.org (Markos Chandras) Upstream: Remote-ID: lightdm ID: launchpad Homepage: https://www.freedesktop.org/wiki/Software/LightDM Location: /usr/portage/x11-misc/lightdm Keywords: 1.18.3:0: amd64 arm ppc ppc64 x86 Keywords: 1.21.5:0: Keywords: 1.22.0:0: ~amd64 ~arm ~arm64 ~ppc ~ppc64 ~x86 License: GPL-3 LGPL-3