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.

For unrecognized options, Clang 3.1 (and ealier?) prints out a
warning, but it isn't a real warning as it doesn't cause a compilation
error with -Werror, so it is impossible to find out reliably if an
option is supported or not. So hardcode the information we know.

Although Microsoft says that SelectionTree control can publish a control
event only on Windows Server 2003 and above, the  custom action seems to
be working under a fully patched Windows XP SP3. Maybe it fails silently
on older Windows XPs, not to mention Windows 2000. I did not test those.

...as checking the lastsynchronized time-stamps can give wrong results if new and old
impl use same user config and old impl is started once more after new impl is installed,
in which case per-user data of bundled exts of new impl would not be regenerated,
leading to inconsistencies.

...otherwise, if they are bundled, their per-user data is not regenerated, leading
to inconsitencies.