Vendor backports/enumerator for superenv

superenv makes use of some features first introduced in Ruby 1.8.7;
rather than rewrite superenv, this backports those features to 1.8.6.

This probably isn't enough for 1.8.2, which is still untested.
Enable superenv on Leopard

I've been testing superenv for several months on Leopard using the
backported 1.8.7 functionality, and it's working very well.

This leaves Tiger the only platform that does not support superenv.