add xref to WRE

git-svn-id: https://svn.r-project.org/R/trunk@66491 00db46b3-68df-0310-9c12-caf00c1e9a41
more on C++ portability

git-svn-id: https://svn.r-project.org/R/trunk@66492 00db46b3-68df-0310-9c12-caf00c1e9a41