sci-chemistry/chemical-mime-data only requires the rsvg-convert tool; this is ALWAYS included in librsvg, even with USE='-tools'. P.S. Should it really be installing old GNOME 2.4 and KDE 3.x support files? Noticed in its configure output that it claims to be... :x
commit bb039b91f2fbed904c85bfd7e879f90f9c118d6e Author: Justin Lecher <jlec@gentoo.org> Date: Sun Feb 19 11:14:34 2017 +0000 sci-chemistry/chemical-mime-data: Drop unnecessary USE dep Gentoo-Bug: https://bugs.gentoo.org/show_bug.cgi?id=609560 Package-Manager: Portage-2.3.3, Repoman-2.3.1 Signed-off-by: Justin Lecher <jlec@gentoo.org> https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=bb039b91f2fbed904c85bfd7e879f90f9c118d6e