Should we make virtualenv POSIX portable?

readlink is not in POSIX, nor is there any real standard for it, so what features it supports is up in the air.

It would be pretty trivial to change create_venv into a python script using os.readlink.

Ok, apparently you can just brew install coreutils to get gnu readlink on a mac, so this isn't necessary.

See https://stackoverflow.com/questions/1055671/how-can-i-get-the-behavior-of-gnus-readlink-f-on-a-mac