Cygwin's pthread_sigmask() implementation appears to be buggy. This
patch works around this problem by using sigprocmask() instead.

This patch is implemented in a general way so it could be used by other
platforms too. If this approach is deemed too risky, then I can work up
a patch that just hacks Python/thread_pthread.h for Cygwin.

Note that I tested this patch against 2.3c1 under Red Hat Linux 8.0 too.

[snip]
And finally, I need someone to regenerate pyconfig.h.in and configure
with the same versions of the autotools that are normally used by
Python.

Neal kindly regenerated pyconfig.h.in and configure for me.