No more warnings for core code on OS X!

At least on my machine =) . I finally got it so that ``configure`` detects support for the %zd format for printf() and sets PY_SIZE_FORMAT_T if successful. That removes all of the warnings spit out by gcc 4 about the format to prinft() and friends having a format of 'int' but being passed something else (size_t, Py_ssize_t, etc.).

I have not applied the patch yet because I am on vacation and thus do not have the time to watch nervously the result of the buildbot come back possibly red or orange because of my change. So I will get to this when I can (might not be until the week of May 8, though).