Bulding a simple extension (the spam example) fails with mingw64.

in modsupport.h, the following is used to detect 64bit, it does not work with mingw64.

#if SIZEOF_SIZE_T != SIZEOF_INT
/* On a 64-bit system, rename the Py_InitModule4 so that 2.4
   modules cannot get loaded into a 2.5 interpreter */
#define Py_InitModule4 Py_InitModule4_64
#endif

This code never compiles, you can test this by placing similar code and filling it with rubbish.

This means it thinks the extension is being built on a 32bit compiler and creates the wrong call for Py_InitModule.

Workaround:
Explicitly calling Py_InitModule4_64() in extension and declaring Py_InitModule4_64(...) in code. Note this does not complain about re-declaration and builds OK because declaration is wrong.
e.g.
//m=Py_InitModule("spam", SpamMethods);
m = Py_InitModule4_64("spam", SpamMethods,(char *)NULL, (PyObject *)NULL, PYTHON_API_VERSION);


Or, a better more portable permanent fix, define WIN64 in code and
modify modsupport.h:
#if SIZEOF_SIZE_T != SIZEOF_INT || defined(WIN64)
/* On a 64-bit system, rename the Py_InitModule4 so that 2.4
   modules cannot get loaded into a 2.5 interpreter */
#define Py_InitModule4 Py_InitModule4_64
#endif

I am sure there are other, more standard ways.

What are the values of SIZEOF_SIZE_T and SIZEOF_INT with this compiler?
I'd expect them to be respectively 8 and 4.

OK,  I've understood this a bit more.

The compiler does not cause pyconfig.h to define SIZEOF_SIZE_T and SIZEOF_INT, rather it is the definition of MS_WIN64 at compile time (which was not being defined)

Defining MS_WIN64 fixes the problem, and causes another unrelated one....

I would guess that I would have to change the patch in Issue11723 to define MS_WIN64, how do I then use the same compiler for 32BIT?  What is the correct way to pass such flags to the compiler class?

pyconfig.h defines SIZEOF_SIZE_T depending on the preprocessor symbol MS_WIN64. The patch in #4709 would fix that.