nix: use -O2 on all packages

Author: mgmeier

nix/haskell.nix

@@ -351,11 +351,14 @@ let
               lib.mkOption {
                 type = lib.types.submodule (
                   { config, lib, ... }:
-                  lib.mkIf config.package.isLocal
-                  {
-                    configureFlags = [ "--ghc-option=-Werror"]
-                      ++ lib.optional (args.config.compiler.version == "8.10.7") "--ghc-option=-Wwarn=unused-packages";
-                  }
+                    lib.mkMerge [
+                      (lib.mkIf config.package.isLocal {
+                        configureFlags = [ "--ghc-option=-Werror" "--ghc-option=-O2" ];
+                      })
+                      (lib.mkIf (!config.package.isLocal) {
+                        configureFlags = [ "--ghc-option=-O2" ];
+                      })
+                    ]
                 );
               });
           })