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.
Is libpng used by Wesnoth itself, or only via sdl_image? If the latter, I'd rather get the issue that sdl_image can pick up the wrong libsdl fixed in Linuxbrew/Homebrew than carry a workaround here.