darwin: add impure-cmds

On darwin, there are some commands neither opensource nor able to build in nixpkgs.
We have no choice but to use those system-shipped impure ones.

pkgs/os-specific/darwin/impure-cmds/default.nix

@@ -0,0 +1,34 @@
+{ lib, runCommandLocal }:
+
+# On darwin, there are some commands neither opensource nor able to build in nixpkgs.
+# We have no choice but to use those system-shipped impure ones.
+
+let
+  commands = {
+    ditto = "/usr/bin/ditto"; # ditto is not opensource
+    sudo  = "/usr/bin/sudo";  # sudo must be owned by uid 0 and have the setuid bit set
+  };
+
+  mkImpureDrv = name: path:
+    runCommandLocal "${name}-impure-darwin" {
+      __impureHostDeps = [ path ];
+
+      meta = {
+        platforms = lib.platforms.darwin;
+      };
+    } ''
+      if ! [ -x ${path} ]; then
+        echo Cannot find command ${path}
+        exit 1
+      fi
+
+      mkdir -p $out/bin
+      ln -s ${path} $out/bin
+
+      manpage="/usr/share/man/man1/${name}.1"
+      if [ -f $manpage ]; then
+        mkdir -p $out/share/man/man1
+        ln -s $manpage $out/share/man/man1
+      fi
+    '';
+in lib.mapAttrs mkImpureDrv commands