jack2: fix build on Darwin

jack_control requires dbus, which isn’t enabled on Darwin.

pkgs/misc/jackaudio/default.nix

@@ -61,7 +61,7 @@ stdenv.mkDerivation (finalAttrs: {
   postInstall = (if libOnly then ''
     rm -rf $out/{bin,share}
     rm -rf $out/lib/{jack,libjacknet*,libjackserver*}
-  '' else ''
+  '' else lib.optionalString (optDbus != null) ''
     wrapProgram $out/bin/jack_control --set PYTHONPATH $PYTHONPATH
   '');