Add support for building Python against GNU Portable Threads (Pth),
and also add experimental support for POSIX semaphores in FreeBSD
7-STABLE and up. The option knobs PTH and SEM respectively are
added to enable this behaviour.
Python is able to use POSIX semaphores for thread synchronization
in threading, and prefers them.
The multiprocessing module in Python 2.6 requires POSIX semaphores,
however, the FreeBSD rtld and malloc need further work to allow
a process to call pthread_create() immediately after fork() as it is
not something allowed by a strict interpretation of the POSIX specs;
therefore allow GNU Pth to be used until the situation is resolved.
Approved by: miwi