Forced -j1 is needed with udev included: https://bugs.gentoo.org/show_bug.cgi?id=451500 Confirmed -j1 fixes the install failure.
Thanks for the report! + 24 Mar 2013; Mike Gilbert <floppym@gentoo.org> systemd-198-r4.ebuild, + systemd-9999.ebuild: + Run the install phase with a single make job, bug 463156 by Ray Griffin.