net-libs/webkit-gtk-2.4.11(-r1) fails to build with GCC 6. See the relevant bug on the upstream bugzilla: https://bugs.webkit.org/show_bug.cgi?id=159124. There are relevant patches from the Fedora project which fix issue attached to the upstream issue.
Pulled in fedora patch for 2.4* versions as: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=bfe97a5ba14edaafe8694f6a74aa026506eacc68