Use PTHREAD_CREATE_DETACHED macro instead of 1: fix Solaris crash

When calling pthread_attr_setdetachstate, we were using 1 as the
argument. But the pthreads documentation says that you have to say
PTHREAD_CREATE_DETACH, which on Solaris is apparently 0x40.  Calling
pthread_attr_setdetachstate with 1 crashes on Solaris with FLTBOUNDS.

(Because we're so late in the release cycle, I made the code define
PTHREAD_CREATE_DETACHED if it doesn't exist, so we aren't likely to
break any other platforms.)

This bug was introduced when we made threading mandatory in
0.2.6.1-alpha; previously, we had force-disabled threading on
Solaris.  See #9495 discussion.