Recipe: centralise dependencies for latin.c.

Recipe: centralise dependencies for latin.c.

It's silly to have every puzzle using latin.c separately specify in
its .R file the list of additional modules that latin.c depends on, or
for that matter to have them all have to separately know how to adjust
that for the STANDALONE_SOLVER mode of latin.c.

So I've centralised a new pair of definitions into the core Recipe
file, called LATIN and LATIN_SOLVER, and now a client of latin.c need
only ask for that to get all the necessary dependencies too.

Also, while I'm here, I've moved the non-puzzle-specific 'latincheck'
test program out of unequal.R into the central Recipe.