The correct answer for Nix is to never link against a library provided by the system. Nix provides its own bootstrap binaries and it must be done that way for the sake of reproducibility.
well, there's xcode-select to chose the SDK you want which kinda implies the availability of multiple – currently for example swift 2.2 and 3.0.
I'm tempted by the "1-environment" idea but homebrew has done such an excellent job in comparison to fink/port, I';; stay loyal. Also wondering if those two projects couldn't profit from some code sharing?
Apple doesn't help this -- they want you to just install xcode and the most recent SDK and nothing else.