Version 0.3.2 of media-libs/lensfun has been released and is required to build media-plugins/gimp-lensfun-0.2.4
Bumped in tree, thanks for the report! https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=609f3bc64037ab3c1e57e8ea068dae22ecf1bd2f