If you can’t update your world set because of circular dependencies,
then you can best comment out your USE-flags in /etc/make.conf and put USE="-*"
underneath it. Then update world and when that succeeds, remove the
latter USE declaration in your make.conf and uncomment your former
USE-flags. Update/rebuild world again and if all goes well you won’t
have a single circular dependency
http://treggats.wordpress.com/2008/10/09/the-story-continues/