See the relevant archlinux bug: https://bugs.archlinux.org/task/49287 This can be fixed by adding `append-cflags -std=c90` to the ebuild.
commit f98f2d1e819cea8085f0c3bbc2771d363f430b72 Author: David Seifert <soap@gentoo.org> Date: Thu Sep 15 09:44:33 2016 +0200 dev-libs/ucl: Allow for compiling with GCC 6 Gentoo-bug: 585632