[ZhongRuoyu]

_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.

Please backport to 3.15.

CC: @encukou

• Issue: Build fails with glibc 2.13 after static_assert fallback regression #160704