sys-power/upower would need this. see latest upower ebuild and configure.ac of it. it's defined in the systemd.pc. it's no way any different than what the unitdir stuff is now. thanks!
/var/cvsroot/gentoo-x86/eclass/ChangeLog,v <-- ChangeLog new revision: 1.487; previous revision: 1.486 /var/cvsroot/gentoo-x86/eclass/systemd.eclass,v <-- systemd.eclass new revision: 1.17; previous revision: 1.16