Please use REQUIRED_USE instead.
commit cb37794e3a4e8205ff54ad5676aad0ed437c6ba6 Author: Michał Górny <mgorny@gentoo.org> AuthorDate: Sun Mar 12 09:05:34 2017 Commit: Michał Górny <mgorny@gentoo.org> CommitDate: Sun Mar 12 10:05:02 2017 www-apps/viewvc: confutils -> REQUIRED_USE, #612394