gh-160704: Restore C11-C17 static_assert fallback

`_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`.

Below is an example when building CPython 3.15.2 with glibc 2.13:

    gcc -pthread -c -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer  -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I/usr/local/src/python/Include/internal -I/usr/local/src/python/Include/internal/mimalloc -IObjects -IInclude -IPython -I. -I/usr/local/src/python/Include    -DPy_BUILD_CORE -o Objects/longobject.o /usr/local/src/python/Objects/longobject.c
    In file included from /usr/local/src/python/Include/Python.h:70,
                    from /usr/local/src/python/Objects/longobject.c:5:
    /usr/local/src/python/Include/internal/pycore_bitutils.h: In function '_Py_popcount32':
    /usr/local/src/python/Include/pymacro.h:175:13: error: implicit declaration of function 'static_assert' [-Wimplicit-function-declaration]
      175 |             static_assert((cond), #cond); \
          |             ^~~~~~~~~~~~~
    /usr/local/src/python/Include/internal/pycore_bitutils.h:100:5: note: in expansion of macro 'Py_BUILD_ASSERT'
      100 |     Py_BUILD_ASSERT(sizeof(x) <= sizeof(unsigned int));
          |     ^~~~~~~~~~~~~~~
    make: *** [Makefile:3407: Objects/longobject.o] Error 1

Restore compilation in such environments by fixing the negation.

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

Author: ZhongRuoyu

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.