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.
Reading my comment again, does this mean I should mark it as => :build?

I guess maybe I might do this when new stable is released and 1.10 is deleted from this formula.