gh-160208: Fix _Py_ANONYMOUS on C++

Use __extension__ on g++/clang++ to fix this g++ warning on
Include/object.h:

    error: ISO C++ prohibits anonymous structs [-Wpedantic]

Author: vstinner

Include/pymacro.h

@@ -95,12 +95,13 @@
 // - MSVC has warning(disable: 4201) "nonstandard extension used : nameless
 //   struct/union". This is specific enough that we disable it for all of
 //   Python.h.
-// - GCC & clang needs __extension__ before C11
+// - GCC & clang needs __extension__ before C11 or on C++.
 // To allow unsupported platforms which need other spellings, we use a
 // predefined value of _Py_ANONYMOUS if it exists.
 #ifndef _Py_ANONYMOUS
 #   if (defined(__GNUC__) || defined(__clang__)) \
-          && !(defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
+          && (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112L \
+              || defined(__cplusplus))
 #       define _Py_ANONYMOUS __extension__
 #   else
 #       define _Py_ANONYMOUS