Merge pull request #218781 from viraptor/ruby-libxml-darwin

rubyPackages.libxml-ruby: fix darwin build

pkgs/development/ruby-modules/gem-config/default.nix

@@ -407,6 +407,9 @@ in
     buildFlags = [
       "--with-xml2-lib=${libxml2.out}/lib"
       "--with-xml2-include=${libxml2.dev}/include/libxml2"
+    ] ++ lib.optionals stdenv.isDarwin [
+      "--with-iconv-dir=${libiconv}"
+      "--with-opt-include=${libiconv}/include"
     ];
   };