- instead on lots of recursive build.pl modules to iterate through we build all
  tail_build up to the requested module inside one tail_build when doing a
  "make foo.all"
- advantage: speeding up because getting rid of lots of recursive makes with
  artificial module barriers, forcing almost sequential execution: build, link,
  unittest with lots of cpu-idling at each module-end
- disadvantage: all shared dependencies of tail_build are now build for a
  (sub)module of tail_build. This is transitional as we migrate more stuff into
  tail_build and there are good dependencies inside tail_build
- "make foo.all" now only executes slowcheck now for foo, not for all the deps