gthree: mark as broken on darwin

pkgs/development/libraries/gthree/default.nix

···
     license = licenses.mit;
     maintainers = with maintainers; [ jtojnar ];
     platforms = platforms.unix;
+    broken = stdenv.isDarwin; # never built on Hydra https://hydra.nixos.org/job/nixpkgs/trunk/gthree.x86_64-darwin
   };
 }