Upstream does supply a set of systemd unit files in the distribution tarball and an ability to control installation of those files through the --with/--without-systemdsystemunitdir option. The ebuild is currently unaware of these options, making installation of systemd units pkgconfig file dependant and thus automagic.
The ebuild shall use systemd.eclass to handle installation of the systemd unit files in a clean and unified way.
The preferred way to handle that is to use autotools-utils eclass, and call `systemd_to_myeconfargs' before `autotools-utils_src_configure', as in:
inherit autotools-utils systemd
If you're not interested in migrating to autotools-utils, there is also a legacy `systemd_with_unitdir' function, which could be used like that:
$(use_enable foo) \
Currently introduction of the eclass will cause the package to install systemd unit files unconditionally. If you don't like that concept, feel free to take a vote on the appropriate gentoo-dev ml topic. The eclass behaviour might change in future.
+*libcanberra-0.28-r2 (08 May 2011)
+ 08 May 2011; Gilles Dartiguelongue <firstname.lastname@example.org>
+ Add support for systemd, bug #366177.
Fixed, thanks for reporting.