Fix build_signature in setoid rewriting which was building an ill-typed term using lift (-1)
and simultaneously avoid making relation evars dependent on the local context when inferring signatures if the 
carrier of the relation is closed.


git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16348 85f007b7-540e-0410-9357-904b9bb8a0f7