Add package separation for nix-darwin users with a macOS Sdk installation present

[haze]

Add nix_include_dirs and nix_framework_dirs for nix-darwin users who have packages installed that aren't relative to an existing SDK root

(this makes it so zig can find and build against c libraries provided by a shell.nix)

[kubkon]

I think we should figure out a way of satisfying nix-darwin without such special-casing like that.

[haze]

All nix users will have this problem. It's just that Darwin users also have an sdk root, so Zig attempts to add the paths in relation to, which is incorrect. It's not special casing it's support.

[haze]

repro:

test_time.c

#include <time.h>
#include <stdio.h>

int main() {
  struct timespec time;
  int status = clock_gettime(CLOCK_REALTIME, &time);
  printf("%ld\n", time.tv_sec);
}

shell.nix

{ pkgs ? import <nixpkgs> {}, pkgs_unstable ? import <nixpkgs-unstable> {}, }:
  pkgs.mkShell {
    nativeBuildInputs = [ 
      pkgs.llvmPackages_13.libclang
      pkgs.darwin.apple_sdk.MacOSX-SDK
    ];
}

(this gives you a library and also a clang to test against that doesn't have the sysroot baked in)