bpo-35081: Internal headers require Py_BUILD_CORE (GH-10363)

* 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.

Include/internal/pycore_pathconfig.h

@@ -4,8 +4,8 @@
 extern "C" {
 #endif
 
-#ifndef Py_BUILD_CORE
-#  error "Py_BUILD_CORE must be defined to include this header"
+#if !defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_BUILTIN)
+#  error "this header requires Py_BUILD_CORE or Py_BUILD_CORE_BUILTIN define"
 #endif
 
 PyAPI_FUNC(void) _Py_wstrlist_clear(