It's deprecated for ages and nothing in the tree is using it: I would drop it The only package setting that variable is gretl, but it's not using gnome2.eclas
Please do.
+ 16 Dec 2014; Pacho Ramos <pacho@gentoo.org> gnome2.eclass: + Deprecate USE_EINSTALL (#482082) + (in a month we should be able to kill it after, hopefully, any people from overlays notice they need to migrate)
23 Feb 2015; Pacho Ramos <pacho@gentoo.org> gnome2.eclass: + Remove USE_EINSTALL (#482082) +