gh-154643: Only use _Py_STDC_VERSION when defined

This fixes:

    $ g++ -I/usr/include/python3.15 -c -Werror=undef test.cpp
    In file included from /usr/include/python3.15/Python.h:68,
                     from test.cpp:1:
    /usr/include/python3.15/pyport.h:494:9: error: ‘_Py_STDC_VERSION’ is not defined, evaluates to ‘0’ [-Werror=undef]
      494 | #  elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
          |         ^~~~~~~~~~~~~~~~
    cc1plus: some warnings being treated as errors

For any test.cpp that has #include <Python.h>.

For locating the place for the added test, I used LLM.

The main issue I could reproduce was in Include/pyport.h.
Include/cpython/pyatomic.h was updated for completeness.

Include/internal/mimalloc/mimalloc/atomic.h also has this
for __STDC_VERSION__
but was left untouched as it is code from a bundled project
and is likely not exposed publicly.

Assisted-By: Claude Opus 4.6

Author: hroncok

Include/cpython/pyatomic.h

@@ -585,7 +585,7 @@ static inline void _Py_atomic_fence_release(void);
 #  define Py_ATOMIC_GCC_H
 #  include "pyatomic_gcc.h"
 #  undef Py_ATOMIC_GCC_H
-#elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_ATOMICS__)
+#elif defined(_Py_STDC_VERSION) && _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_ATOMICS__)
 #  define Py_ATOMIC_STD_H
 #  include "pyatomic_std.h"
 #  undef Py_ATOMIC_STD_H

Include/pyport.h

@@ -491,7 +491,7 @@ extern "C" {
 #ifdef WITH_THREAD
 #  ifdef thread_local
 #    define _Py_thread_local thread_local
-#  elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
+#  elif defined(_Py_STDC_VERSION) && _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
 #    define _Py_thread_local _Thread_local
 #  elif defined(_MSC_VER)  /* AKA NT_THREADS */
 #    define _Py_thread_local __declspec(thread)

Lib/test/test_cext/__init__.py

@@ -174,6 +174,11 @@ def test_build_abi3t(self):
         # Test with Py_TARGET_ABI3T
         self.check_build('_test_cppext_abi3t', abi3t=True)
 
+    # gh-154643: Test that headers don't use undefined macros in #if
+    @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support -Werror=undef")
+    def test_build_werror_undef(self):
+        self.check_build('_testcppext_undef', extra_cflags='-Werror=undef')
+
 
 class TestInternalC(BaseTests, unittest.TestCase):
     LANGUAGE = 'C'

Misc/NEWS.d/next/C_API/2026-07-24-22-30-40.gh-issue-154643.z-ZutK.rst

@@ -0,0 +1,2 @@
+In ``pyport.h`` and ``pyatomic.h`` don't use ``_Py_STDC_VERSION`` when it is
+not defined, fixing build errors for C++ code built with ``-Werror=undef``.