Various fixes to streamline build process on Mac OS X:

Various fixes to streamline build process on Mac OS X:
- Give a warning if you're on a case-insensitive filesystem and have
  not specified --with-suffix.
- Don't require --with-dyld, it is now default for OSX/Darwin (suggested
  by Martin v. Loewis)
- Don't define _POSIX_THREADS on Darwin, it's done by standard headers already
  (fix by Tony Lownds)
- Don't use the Mac subtree anymore, the routines relevant to OSX/Darwin
  have moved to a new file Python/mactoolboxglue.c.