Improve threading on Solaris, according to SF patch #460269, submitted

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.