Move bulk of rules from Makefile to Makefile.local.

Before it was impossible to know whether any particular setting or
rule definition was in Makefile or Makefile.local. So we strip the
Makefile down to little more than the list of sub-directories and
the logic to include all of the sub-directories' Makefile.local
fragments.

Then, all of the real work can happen inside of Makefile.local.