c: stddef.h C23 fixes [PR114870]

c: stddef.h C23 fixes [PR114870]

The stddef.h header for C23 defines __STDC_VERSION_STDDEF_H__ and
unreachable macros multiple times in some cases.
The header doesn't have normal multiple inclusion guard, because it supports
for glibc inclusion with __need_{size_t,wchar_t,ptrdiff_t,wint_t,NULL}.
While the definition of __STDC_VERSION_STDDEF_H__ and unreachable is done
solely in the #ifdef _STDDEF_H part, so they are defined only if stddef.h
is included without those __need_* macros defined.  But actually once
stddef.h is included without the __need_* macros, _STDDEF_H is then defined
and while further stddef.h includes without __need_* macros don't do
anything:
 #if (!defined(_STDDEF_H) && !defined(_STDDEF_H_) && !defined(_ANSI_STDDEF_H) \
      && !defined(__STDDEF_H__)) \
     || defined(__need_wchar_t) || defined(__need_size_t) \
     || defined(__need_ptrdiff_t) || defined(__need_NULL) \
     || defined(__need_wint_t)
if one includes whole stddef.h first and then stddef.h with some of the
__need_* macros defined, the #ifdef _STDDEF_H part is used again.
It isn't that big deal for most cases, as it uses extra guarding macros
like:
 #ifndef _GCC_MAX_ALIGN_T
 #define _GCC_MAX_ALIGN_T
 ...
 #endif
etc., but for __STDC_VERSION_STDDEF_H__/unreachable nothing like that is
used.

So, either we do what the following patch does and just don't define
__STDC_VERSION_STDDEF_H__/unreachable second time, or use #ifndef
unreachable separately for the #define unreachable() case, or use
new _GCC_STDC_VERSION_STDDEF_H macro to guard this (or two, one for
__STDC_VERSION_STDDEF_H__ and one for unreachable), or rework the initial
condition to be just
 #if !defined(_STDDEF_H) && !defined(_STDDEF_H_) && !defined(_ANSI_STDDEF_H) \
     && !defined(__STDDEF_H__)
- I really don't understand why the header should do anything at all after
it has been included once without __need_* macros.  But changing how this
behaves after 35 years might be risky for various OS/libc combinations.

2025-02-26  Jakub Jelinek  <jakub@redhat.com>

	PR c/114870
	* ginclude/stddef.h (__STDC_VERSION_STDDEF_H__, unreachable): Don't
	redefine multiple times if stddef.h is first included without __need_*
	defines and later with them.  Move nullptr_t and unreachable and
	__STDC_VERSION_STDDEF_H__ definitions into the same
	defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L #if block.

	* gcc.dg/c23-stddef-2.c: New test.