nixos/xmonad: allow passing compile time options to ghc invocation

nixos/modules/services/x11/window-managers/xmonad.nix

@@ -16,6 +16,7 @@ let
                 cfg.extraPackages cfg.haskellPackages ++
                 optionals cfg.enableContribAndExtras
                 (with cfg.haskellPackages; [ xmonad-contrib xmonad-extras ]);
+    inherit (cfg) ghcArgs;
   } cfg.config;
 
 in
@@ -85,6 +86,15 @@ in
         '';
       };
 
+      ghcArgs = mkOption {
+        default = [];
+        type = with lib.types; listOf str;
+        description = ''
+          Command line arguments passed to the compiler (ghc)
+          invocation when xmonad.config is set.
+        '';
+      };
+
     };
   };
   config = mkIf cfg.enable {