python3Packages.multidict: fixup build on darwin

https://hydra.nixos.org/build/304989971/nixlog/4/tail

pkgs/development/python-modules/multidict/default.nix

@@ -1,4 +1,5 @@
 {
+  stdenv,
   lib,
   fetchFromGitHub,
   buildPythonPackage,
@@ -35,6 +36,10 @@ buildPythonPackage rec {
     typing-extensions
   ];
 
+  env = {} // lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = "-Wno-error=unused-command-line-argument";
+  };
+
   nativeCheckInputs = [
     objgraph
     pytestCheckHook