The dev-lang/v8 ebuild does use EXTRA_ESCONS which is (much like EXTRA_EMAKE) not supposed to be used within ebuilds. Please pass the appropriate parameters directly instead. This is my oversight that this wasn't mentioned in eclass docs before.
Thanks for the report. I fixed the problem in all ebuilds except the stable one. If that's enough, feel free to close this bug now.
Removed last v8 ebuild using EXTRA_ESCONS from the tree, closing.