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.
Sorry, I'm actually referring to the `-j` flag. Homebrew automatically sets `MAKEFLAGS`, which sets the number of jobs.

Otherwise, you can pass the number of make jobs that Homebrew determines: `"-j #{ENV.make_jobs}"`