After upgrading to sys-libs/glibc-2.26, sys-fs/zfs will fail to configure with the error "xdrmem_create() requires tirpc or libc". This is due to SunRPC being removed from sys-libs/glibc-2.26. See <https://wiki.gentoo.org/wiki/Project:Toolchain/RPC_implementation> As a result, net-libs/libtirpc needs to be installed before sys-fs/zfs can configure and compile.
Thanks James. I added an explicit dependency to all sys-fs/zfs ebuilds for net-libs/libtirpc. The commit can be found here: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=df1cc0f5f518b877bc29d7b0fa15456203041d0f