Imported Boehm GC 5.0 release

[Bryce McKinlay]

I've imported the 5.0 release of the collector. Again, the changes are
minor, but now we can say we're not running an "alpha" release!

regards

  [ bryce ]

Since 5.0alpha7:
 - Fixed threadlibs.c for linux threads.  -DUSE_LD_WRAP was
    broken and -ldl was omitted.  Fixed Linux stack finding code to
    handle -DUSE_LD_WRAP correctly.
 - Added MSWIN32 exception handler around marker, so that the
    collector can recover from root segments that are unmapped
    during the collection. This caused occasional failures under
    Windows 98, and may also be an issue under Windows NT/2000.