In 3.15, upstream has re-introduced the dillo plugin as an alternative to webkit for reading html mail. However, the gentoo 3.15.0 ebuild does not support dillo. Please add support and an USE flag for dillo!
Sorry but NEWS file from claws-mail-3.15.0 says the opposite: plugins have been dropped: TrayIcon, Dillo Viewer, and Gtkhtml2 Viewer. And indeed, looking at configure.ac file, the dillo-plugin switch has been commented out and commenting it back in does not build dillo plugin. So unfortunately there's nothing that can be done here other than convincing upstream to re-enable it.
Sorry, I didn't check the release, but there have definitely been dillo commits in the claws repository recently: http://git.claws-mail.org/?p=claws.git;a=commit;h=1e068dbfc2837809030dcbf0173d2841a411d95a http://git.claws-mail.org/?p=claws.git;a=commit;h=2bf719fcbf4e1e2cbfb0b574db48c468375e4dbf Can we take them?
claws-mail upstream recommends to not introduce the plugin yet. It's still considered being in developemnt. That's the reason why they did not add it to the release tarball (despite being in the 3.15.0 tagged git tree).
dillo plugin is back in claws-mail-3.17.0 and the ebuild was changed accordingly.