Envar: nicer code to add .exe when searching for caml binaries in win32

 Btw, the previous patch was contributed by Pierr-Yves Strub

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16125 85f007b7-540e-0410-9357-904b9bb8a0f7