[3.15] gh-160704: Restore C11-C17 static_assert fallback (GH-160705) (GH-160825)

gh-160704: Restore C11-C17 `static_assert` fallback (GH-160705)

`_Py_STDC_VERSION` needs to be defined before its value can be checked.
In 8335bcb (gh-160208: Add a private wrapper around __STDC_VERSION__
(GH-160376), 2026-10-23), the condition `defined(_Py_STDC_VERSION)` is
incorrectly negated, which prevents the fallback definition of
`static_assert` in C11-C17 builds and causes a build error when building
with an old C library that does not define `static_assert`.
(cherry picked from commit a7ac7a8)

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>
Co-authored-by: Ruoyu Zhong <zhongruoyu@outlook.com>

Author: miss-islington

Include/pymacro.h

@@ -25,7 +25,7 @@
 // behaviour. So only define if building as C, not C++ (if __cplusplus is
 // not defined), and only for C11-17.
 #if !defined(static_assert) && (defined(__GNUC__) || defined(__clang__)) \
-     && !defined(_Py_STDC_VERSION) \
+     && defined(_Py_STDC_VERSION) \
      && _Py_STDC_VERSION >= 201112L && _Py_STDC_VERSION <= 201710L
 #  define static_assert _Static_assert
 #endif

Misc/NEWS.d/next/Build/2026-10-26-21-00-00.gh-issue-160704.ZRYLKF.rst

@@ -0,0 +1,2 @@
+Fix C11-C17 builds with old C library headers that do not define
+``static_assert``, including glibc versions older than 2.16.