PEP353 recommends to add the following code snippet to extension 
modules to ensure compatibility with pre-2.5 interpreters:

#if PY_VERSION_HEX < 0x02050000
typedef int Py_ssize_t;
#define PY_SSIZE_T_MAX INT_MAX
#define PY_SSIZE_T_MIN INT_MIN
#endif

This is insufficient, because type definitions that use Py_ssize_t must 
also be added. Here is what works for me, though they may be more 
definitions to be included:

#if PY_VERSION_HEX < 0x02050000
typedef int Py_ssize_t;
#define PY_SSIZE_T_MAX INT_MAX
#define PY_SSIZE_T_MIN INT_MIN
typedef Py_ssize_t (*lenfunc)(PyObject *);
typedef PyObject *(*ssizeargfunc)(PyObject *, Py_ssize_t);
typedef PyObject *(*ssizessizeargfunc)(PyObject *, Py_ssize_t, 
Py_ssize_t);
typedef int(*ssizeobjargproc)(PyObject *, Py_ssize_t, PyObject *);
typedef int(*ssizessizeobjargproc)(PyObject *, Py_ssize_t, Py_ssize_t, 
PyObject *);
typedef Py_ssize_t (*readbufferproc)(PyObject *, Py_ssize_t, void **);
typedef Py_ssize_t (*writebufferproc)(PyObject *, Py_ssize_t, void **);
typedef Py_ssize_t (*segcountproc)(PyObject *, Py_ssize_t *);
typedef Py_ssize_t (*charbufferproc)(PyObject *, Py_ssize_t, char **);
#endif

However, the main problem is elsewhere. Extension modules may well 
need to use Py_ssize_t in their own data types, and may furthermore 
with to make these data types available to yet other extension 
modules. In practice, this requires the inclusion of a code block such 
as the one shown above inside header files. However, this requires a 
mechanism for avoiding redefinitions, which at the moment does not 
seem to exist. My request is to add such a mechanism to Python 2.5 
and recommend its consistent use in an update of PEP353.

Concretely, I propose that Python 2.5 should define a macro 
PY_HAS_PY_SSIZE_T, and that PEP353 should recommend the inclusion 
of the code snippet

#ifndef PY_HAS_PY_SSIZE_T
#define PY_HAS_PY_SSIZE_T
typedef int Py_ssize_t;

/* add all the other definitions here */

#endif

that contains an exhaustive set of definitions that depend on 
Py_ssize_t.