During configure phase of bluez-5.18 emerge (with udev use flag), the process fails at "checking udev directory... configure: error: udev directory is required". It is because configure script expects udev path in the "with_udevdir" argument or configured "udevdir" variable in /usr/share/pkgconfig/udev.pc. Where is "udevdir" variable in udev.pc installed by systemd-212-r3, but it has not value. Reproducible: Always For a workaround user can disable udev flag or specify udevdir in /usr/share/pkgconfig/udev.pc (udevdir=/lib/udev in my case).
*** This bug has been marked as a duplicate of bug 509454 ***