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.
That option's actually off by default; the user has to explicitly opt into it. Specifying the dep this way will actually prompt the user to uninstall sdl and reinstall without that option if it's already installed.