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.
Not in the same formula, no. We don't have an `oldstable` spec like there is a development one. If an old stable branch needs a formula, it can be spun off into a new formula like in homebrew/versions (which e.g. has postgres9, postgres91, postgres92).