gh-91321: Fix test_cppext for C++03 (GH-93902) (#93904)

Don't build _testcppext.cpp with -Wzero-as-null-pointer-constant when
testing C++03: only use this compiler flag with C++11.
(cherry picked from commit a38c2a6)

Co-authored-by: Victor Stinner <vstinner@python.org>

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Lib/test/setup_testcppext.py

@@ -19,8 +19,6 @@
         '-Werror',
         # Warn on old-style cast (C cast) like: (PyObject*)op
         '-Wold-style-cast',
-        # Warn when using NULL rather than _Py_NULL in static inline functions
-        '-Wzero-as-null-pointer-constant',
     ]
 else:
     # Don't pass any compiler flag to MSVC
@@ -39,6 +37,10 @@ def main():
         name = '_testcpp11ext'
 
     cppflags = [*CPPFLAGS, f'-std={std}']
+    if std == 'c++11':
+        # Warn when using NULL rather than _Py_NULL in static inline functions
+        cppflags.append('-Wzero-as-null-pointer-constant')
+
     cpp_ext = Extension(
         name,
         sources=[SOURCE],