A bit hacky / confusing but rewriting this as

  ${if stdenv.hostPlatform.isDarwin then "NIX_LDFLAGS" else null } = "-lobjc";

should avoid a Linux rebuild for now if that's a concern here.

Hehe, that's neat, didn't actually know it was possible to circumvent the problem this way (though that should ideally be the meaning of foo = null tbh.).

If we want to target master then I think we do want this hack, preferably with a FIXME comment.

I'm inclined to the initial version, because is more easy to understand; but I don't mind updating it. Please let me know if we are going to use this approach

Yours' definitely much clearer so I would also be in favour of that. I think I may have left my comment at the time there was still a change triggering a Linux rebuild, as that's no longer an issue so you can just dismiss my comment.

Right, I guess optionalString returns an empty string when false (which does make sense), but foo = null actually works the way we want. That makes it a lot more palatable indeed :)