[committed] c: Default to -std=gnu23

[Florian Weimer]

* Joseph Myers:

> Change the default language version for C compilation from -std=gnu17
> to -std=gnu23.  A few tests are updated to remove local definitions of
> bool, true and false (where making such an unconditional test change
> seemed to make more sense than changing the test conditionally earlier
> or building it with -std=gnu17); most test issues were already
> addressed in previous patches.  In the case of
> ctf-function-pointers-2.c, it was agreed in bug 117289 that it would
> be OK to put -std=gnu17 in the test and leave more optimal BTF / CTF
> output for this test as a potential future improvement.
>
> Since the original test fixes, more such fixes have become necessary
> and so are included in this patch.  More noinline attributes are added
> to simulate-thread tests where () meaning a prototype affected test
> results, while gcc.dg/torture/pr117496-1.c (a test declaring a
> function with () then calling it with arguments) gets -std=gnu17
> added.
>
> Bootstrapped with no regressions for x86_64-pc-linux-gnu.

Has anyone performed experiments to determine the impact of this change
on typical free software distributions?

Thanks,
Florian