Since Sep. 22 (@85fade1e, https://github.com/systemd/systemd/commit/85fade1e), sys-apps/systemd-9999 requires >=sys-apps/util-linux-2.27. Otherwise it'll fail to build: checking pkg-config is at least version 0.9.0... yes checking for BLKID... yes checking for MOUNT... no configure: error: *** libmount support required but libraries not found
Filed a PR (https://github.com/gentoo/gentoo/pull/122)