See bug 504116.
+ 21 Dec 2014; Michał Górny <mgorny@gentoo.org> files/xcb-rebuilder.sh: + Use sys-apps/gentoo-functions in xcb-rebuilder, bug #504420. Since this script is only used manually when upgrading to libxcb-1.4 (ancient), I didn't put much effort into it. We may probably remove it along with the upgrade guide.