Glibc 2.26 breaks compilation of all but the most recent revisions of gcc in each major branch due to the namespace change from ucontext --> ucontext_t.

There is a fix in 5.5.0 which has been backported to 5.4.0 in the gentoo gcc-5.4.0-patches-1.8.tar.bz2, which is called out by gentoo's gcc-5.4.0-r4.ebuild.

Without this fix, it is impossible to emerge -e @world when glibc-2.26 is installed.