Log Message:
Update to 6.8.

Changes since 6.7:
 - Added some support for Dragonfly BSD.  (Thanks to Joerg Sonnenberger and
   Thomas Klausner.)
 - Improvements to the HP/UX section of configure.in.
   (Thanks to Andreas Tobler.)
 - GC_unix_get_mem could neglect to release the malloc lock on Irix, under
   extremely unlikely circumstances.  Thanks to Jean-Baptiste Nivois for
   some careful code reading.
 - Added support for kFreeBSD + glibc (Thanks to Petr Salinger)
 - Fix more MacOS threads memory leaks (Thanks to Allan Hsu)
 - Added initial Solaris/X86-64 support (Thanks to Rainer Orth)
 - Applied a long-lost MINGW patch from Gerard Allan for malloc redirection
   with threads.  This one probably makes no sense for 7.0, and was not applied
   there.
 - The Solaris/SPARC definition of GC_INIT() in gc.h wasn't C++-compilable.