Implement C23 const-preserving standard library macros

[Andreas Schwab]

On Jan 19 2026, Joseph Myers wrote:

> This is all conditional on __GLIBC_USE (ISOC23), just like other C23 
> features.  So if you're using std=gnu17 and not using _GNU_SOURCE or 
> _ISOC23_SOURCE (or _ISOC2X_SOURCE or _ISOC2Y_SOURCE) you shouldn't get 
> this feature.

_GNU_SOURCE should not enable this by itself, since it is not a proper
extension.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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