fox: fix darwin builds (#355040)

pkgs/development/libraries/fox/default.nix

@@ -1,16 +1,18 @@
-{ lib
-, stdenv
-, fetchurl
-, libpng
-, libjpeg
-, libtiff
-, zlib
-, bzip2
-, mesa_glu
-, libXcursor
-, libXext
-, libXrandr
-, libXft
+{
+  lib,
+  stdenv,
+  fetchurl,
+  libpng,
+  libjpeg,
+  libtiff,
+  zlib,
+  bzip2,
+  mesa_glu,
+  libXcursor,
+  libXext,
+  libXrandr,
+  libXft,
+  cups,
 }:
 
 stdenv.mkDerivation rec {
@@ -22,7 +24,22 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-bu+IEqNkv9OAf96dPYre3CP759pjalVIbYyc3QSQW2w=";
   };
 
-  buildInputs = [ libpng libjpeg libtiff zlib bzip2 mesa_glu libXcursor libXext libXrandr libXft ];
+  buildInputs =
+    [
+      libpng
+      libjpeg
+      libtiff
+      zlib
+      bzip2
+      mesa_glu
+      libXcursor
+      libXext
+      libXrandr
+      libXft
+    ]
+    ++ lib.optionals stdenv.hostPlatform.isDarwin [
+      cups
+    ];
 
   doCheck = true;