This patch partly (some stuff went in already) ports Python to Monterey.

- Fix bug in thread_pthread.h::PyThread_get_thread_ident() where
  sizeof(pthread) < sizeof(long).
- Add 'configure' for:
	- SIZEOF_PTHREAD is pthread_t can be included via <pthread.h>
	- setting Monterey system name
	- appropriate CC,LINKCC,LDSHARED,OPT, and CCSHARED for Monterey
- Add section in README for Monterey build