bpo-45893: Add missing extern C to initconfig.h (GH-29761)

Co-authored-by: Steve Dower <steve.dower@python.org>
(cherry picked from commit f4afc53bf6)

Co-authored-by: Christian Heimes <christian@python.org>

.github/workflows/build.yml

@@ -115,6 +115,8 @@ jobs:
     runs-on: windows-latest
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
+    env:
+       IncludeUwp: 'true'
     steps:
     - uses: actions/checkout@v2
     - name: Build CPython
@@ -129,6 +131,8 @@ jobs:
     runs-on: windows-latest
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
+    env:
+       IncludeUwp: 'true'
     steps:
     - uses: actions/checkout@v2
     - name: Register MSVC problem matcher

Include/cpython/initconfig.h

@@ -1,6 +1,9 @@
 #ifndef Py_PYCORECONFIG_H
 #define Py_PYCORECONFIG_H
 #ifndef Py_LIMITED_API
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /* --- PyStatus ----------------------------------------------- */
 
@@ -239,5 +242,8 @@ PyAPI_FUNC(PyStatus) PyConfig_SetWideStringList(PyConfig *config,
    See also PyConfig.orig_argv. */
 PyAPI_FUNC(void) Py_GetArgcArgv(int *argc, wchar_t ***argv);
 
+#ifdef __cplusplus
+}
+#endif
 #endif /* !Py_LIMITED_API */
 #endif /* !Py_PYCORECONFIG_H */