> There's PyLong_GetInfo ...

Thanks, I understand that this information can be found at runtime. However, in order to correctly compile C code against a given CPython runtime, there needs to be a guarantee that extension module builds use the same configuration as the CPython build.

> Misconfiguration

I searched some more, and it looks like this was already reported almost ten years ago in issue4709.

The work-around there is to pass -DMS_WIN64, which apparently is not defined by MinGW but required by the CPython headers.