openmw-tes3mp: fix build

pkgs/games/openmw/default.nix

···
-{ stdenv, mkDerivation, fetchFromGitHub, qtbase, openscenegraph, mygui, bullet, ffmpeg
+{ stdenv, mkDerivationWith, fetchFromGitHub, qtbase, openscenegraph, mygui, bullet, ffmpeg
 , boost, cmake, SDL2, unshield, openal, libXt, pkgconfig }:
 
 let
···
       sha256 = "0admnllxic6dcpic0h100927yw766ab55dix002vvdx36i6994jb";
     };
   });
-in mkDerivation rec {
+in mkDerivationWith stdenv.mkDerivation rec {
   version = "0.45.0";
   pname = "openmw";
 

pkgs/games/openmw/tes3mp.nix

···
     description = "Multiplayer for TES3:Morrowind based on OpenMW";
     homepage = https://tes3mp.com/;
     license = licenses.gpl3;
-    platforms = platforms.linux;
+    platforms = [ "x86_64-linux" "i686-linux" ];
     maintainers = with maintainers; [ gnidorah ];
   };
 })

pkgs/top-level/all-packages.nix

···
 
   openmw = libsForQt5.callPackage ../games/openmw { };
 
-  openmw-tes3mp = libsForQt5.callPackage ../games/openmw/tes3mp.nix { };
+  openmw-tes3mp = libsForQt5.callPackage ../games/openmw/tes3mp.nix {
+    openmw = openmw.override {
+      stdenv = gcc8Stdenv;
+      openscenegraph = openscenegraph.override { stdenv = gcc8Stdenv; };
+      mygui = mygui.override { stdenv = gcc8Stdenv; };
+    };
+  };
 
   openraPackages = import ../games/openra pkgs;