Boehm GC memory leak on Darwin

[Sandro Tolaini]

I'm sending this note again since I have been caught in this bug once  
more. Under Darwin (PPC and Intel) there is a serious memory leak at  
every Boehm GC cycle. See discussion and patch here:

http://www.hpl.hp.com/hosted/linux/mail-archives/gc/2005-December/ 
001071.html

I think that the patch should be applied to every maintained branch.

Please maintainers, check this in!

Cheers,
  Sandro Tolaini