backport of solaris thread patch, adding PTHREAD_SCOPE_SYSTEM support:

backport of solaris thread patch, adding PTHREAD_SCOPE_SYSTEM support:

  Improve threading on Solaris, according to SF patch #460269, submitted
  by bbrox@bbrox.org / lionel.ulmer@free.fr.

  This adds a configure check and if all goes well turns on the
  PTHREAD_SCOPE_SYSTEM thread attribute for new threads.

  This should remove the need to add tiny sleeps at the start of threads
  to allow other threads to be scheduled.

This is a semi-feature, but makes such a huge difference to the
performance of Zope on Solaris that it's worthwhile (well, imho).