Dovecot is missing an USE value in the IUSE variable. net-mail/dovecot-0.99.10.4 utilizes the gnutls USE variable. Thus I think gnutls should also be in $IUSE.
Fixed in dovecot-0.99.10.5 but not in dovecot-0.99.10.4 which is still in portage.
Thanks! Fixed in CVS.