Move some variable assignments from Makefile.local to Makefile.config

There's not any special configure logic for determining these variable
values, but if we did add some in the future, then these will now be
in the right place for that.

Additionally, this now makes Makefile.local the single place for the
user to look for manually tweaking a variable assignment, (say, for a
compiler that can't accept a particular warning argument).

With this change, there should rarely be any need for a user to poke
into any Makefile.local file.