Add compatibility with Python 2.4

Python 2.4's include files do not expose the 'Py_ssize_t' type, causing the build process for py-setproctitle to fail.

This pull request contains code to expose the type if building for Python 2.4, based on the definition of the 'Py_ssize_t' type found in Python 3.3's 'include\pyport.h' file.

With this change, py-setproctitle builds, installs, and runs successfully under Python 2.4.