opendungeons: link with pthreads explicitly

pkgs/games/opendungeons/default.nix

@@ -15,6 +15,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ cmake ogre cegui boost sfml openal ois ];
+  NIX_LDFLAGS = [
+    "-lpthread"
+  ];
 
   meta = with stdenv.lib; {
     description = "An open source, real time strategy game sharing game elements with the Dungeon Keeper series and Evil Genius.";