I felt it was time to give full-blown desktop environments a try again so I added the gnome mix-in to my epro current|x86_64|workstation|no-systemd setup. Attempting to build gnome (or even just rebuild world) results in a conflict between gnome <stuff> and polkit, due to different requirements for dev-lang/spidermonkey versions from each.
sys-auth/polkit-0.113 depends on >=dev-lang/spidermonkey-1.8.5:0
Various gnome libraries (dev-libs/gjs is the current culprit, but it looks like others demand the same, I think), are pulling in dev-lang/spidermonkey-24.2.0-r2
Attempting to merge spidermonkey 24 over 1.8.5 fails due to file conflicts on /usr/bin/js and /usr/bin/js-config, which causes portage to back out of the process rather than auto-clobbering the existing files.
You can manually rip out (unmerge) spidermonkey 1.8.5 to force 24 to install (aware this is a bad plan, but SCIENCE! and unhealthy curiousity demanded my actions), which leaves /usr/lib64/libmozjs185.so.1.0 and /usr/lib64/libmozjs185.so.1.0.0 in place for polkit (who then tries to re-merge spidermonkey 1.8.5 over 24 when polkit is rebuilt, which has the same file conflicts as trying 24 on top of 1.8.5), and even after successfully merging spidermonkey 24 in this manner gnome libraries still fail to build due to 'missing mozjs-24 package'-related messages.
Build failures attached for spidermonkey 24 and gjs (post manual -spidermonkey-1.8.5,+24 stupid pet tricks described above).
I believe http://forums.funtoo.org/topic/648-gnome-and-polkit-require-different-dev-langspidermonkey-versions-that-cant-be-installed-at-the-same-time/# is the same issue, so at least one other producer so far.