Hello! It looks like wayland should be included in IUSE_SERVERS, to provide the ability to compile xorg-server only for Xwayland. Right now it's possible only with +minimal, which is not working for many programs, I think.
Thanks for the report! Fixed by commit 922294d422228da8f4331e96482d9c7366a1ae76 Author: Matt Turner <mattst88@gentoo.org> Date: Sun Mar 12 19:08:48 2017 -0700 x11-base/xorg-server: Move wayland to IUSE_SERVERS. Bug: https://bugs.gentoo.org/612452