A working version of Python 2.7.

Confession: I'm stumbling around Nix like a drunken monkey, so this may not be The Right Way To Do It. Nevertheless, this patch gets python 2.7 working from the pure-darwin branch. And with it, a lot of packages that depend on python. So I figure it's worth sending the details back upstream... :-)
Probably the `--without-gcc` flag should be part of the string guarded by `optionalString isDarwin`?