update formula for wesnoth

The new formula uses scons instead of cmake, as that is much
better supported by us (our cmake recipe is sometimes not well
maintained.)

It also adds some missing dependencies, and is fixed to use the
keg-only gettext.
Okay. I'd rather not make another formula right now though, there will be a new stable release fairly soon so I would wait until after that. I guess homebrew doesn't often have oldstable versions? If it's discouraged then I can just drop it.