The current ebuild files for media-libs/imlib2 miss $DEPEND inside RDEPEND, so all deps are considered build-time-deps only.
Actually, if RDEPEND is missing from an ebuild, its default value is ${DEPEND}. From PMS section 8.4: "If RDEPEND is unset (but not if it is set to an empty string) in an ebuild, the package manager must set its value to be equal to the value of DEPEND."
Sorry for the noise. A dep of imlib2 was missing in my emerge -puDN world output. But it's because of the same bugreport I made for kipi-plugins, this does not even pull in imlib2. So this report is obsolete.