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.
I want our users to be able to install without being told they need to install Xquartz (because they don't.) It seems that the default option will result in such prompts. (Is there a better way?)