Sorry about the confusion, "-f" isn't supported either that's why I noticed there is a problem.

$ readlink -f .
readlink: illegal option -- f
usage: readlink [-n] [file ...]

Again, why is does does have to be a shell script anyway? 

I really don't like the propect of having to reimplement the logic in _osx_support in shell code...  I've written loads of large, portable shell scripts in the past (when portable meant supporting a dozen or so different unix flavors), and that's not really an experience worth repeating :-/