openldap: fix build on darwin

I have changed `openldap` so that it uses `clangStdenv` when building on darwin – this fixes issues I'm having on OS X 10.10, but I haven't been able to test it on OS X 10.9.

If merged, this shouldn't break anything, but it would be awesome if someone could test on 10.9. Otherwise, I will try to get a 10.9 machine up and running and test on that.