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 tested this, it appears that -j# arguments to brew work for formulas using make but are ignored by scons.

I also couldn't find this option documented in brew anywhere, either in brew --help or man brew. Is that worth reporting to the main homebrew issue tracker?