nheko && mtxclient: enable for Darwin but mark as broken

pkgs/development/libraries/mtxclient/default.nix

@@ -52,10 +52,9 @@ stdenv.mkDerivation rec {
     homepage = "https://github.com/Nheko-Reborn/mtxclient";
     license = licenses.mit;
     maintainers = with maintainers; [ fpletz ];
-    platforms = platforms.unix;
-
-    # As of 2019-06-30, all of the dependencies are available on macOS but the
-    # package itself does not build.
-    broken = stdenv.isDarwin;
+    platforms = platforms.all;
+    # Should be fixable if a higher clang version is used, see:
+    # https://github.com/NixOS/nixpkgs/pull/85922#issuecomment-619287177
+    broken = stdenv.targetPlatform.isDarwin;
   };
 }