boost::ptr_vector::erase() will delete the element, but fortunately there
is a transfer() method that can be used here.

Had to use a class for the typedef declaration because the data
structure is referenced in other header files.

Just wasted half an hour trying to track down a crash in sc unit test
that was apparently caused by different object files being compiled
with different OSL_DEBUG_LEVELs and thus some having XclDebugObjCounter
and some not.

- 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

Or maybe it's the Mach-O object file format that lacks the
functionality? Anyway, let's try to manage without visibility then.