* All internal header files now require Py_BUILD_CORE or
Py_BUILD_CORE_BUILTIN to be defined.
* _json.c is now compiled with Py_BUILD_CORE_BUILTIN to access
pycore_accu.h header.
* Add an example to Modules/Setup to show how to build _json
as a built-in module; it requires non trivial compiler options.
The accu.h header is no longer part of the Python C API: it has been
moved to the "internal" headers which are restricted to Python
itself.
Replace #include "accu.h" with #include "pycore_accu.h".