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.
This isn't doing what you expect - `:x11` is a symbol (an interned string), so it always evaluates to true. It's not representing whether X11 is installed.