Created attachment 311115 [details] build log
Created attachment 311117 [details] emerge --info output
It appears that you had manually unmasked media-libs/cogl-1.10 and media-libs/clutter-1.10. Don't do that unless you are prepared to deal with breakage. In particular, mutter-3.2 is incompatible with cogl-1.10 and clutter-1.10; you need mutter-3.4 (currently in the gnome overlay, should be arriving masked in portage sometime soon).