haxe: Use ignoreExternals and fix build.

Now, our builds shouldn't break anymore once there is a new change in ocamllibs.

I've used revision 256 from ocamllibs, because this was approximately the
revision we had back then when Haxe 2.10 got released.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>