AFIAK, it's fine to leave the user/pw of the "worker" there, as that's only what connects to the pool and has no transacting abilities... it actually behooves you too, as if someone were to connect; they would be helping you mine.  Either way, I admire your willingness to remove the login/pw - many don't.

Now onto your issue.

I had the same problem, have you tried installing libidn through MacPorts?

If you use homebrew, it's likely just using the wrong directory and you can modify them as seen here.

Hope this helps.