Created attachment 381524 [details] emerge --info =net-libs/gnutls-2.12.23-r6::gentoo gnutls fails to buil on aclocal
Created attachment 381526 [details] /var/tmp/portage/net-libs/gnutls-2.12.23-r6/temp/aclocal-1.out
Given how similar this sounds to bug 517856, there seems there might be something subtly broken in the autotools eclass or in portage. Chances are it's a race condition based bug, so in a few attempts you'll get around it, though the problem will remain.
After continuing emerge -NuDav world by emerge --resume --exclude="gnutls" I run again emerge gnutls and it emerged without errors
should be fixed; see bug 529404 for details