More C type errors by default for GCC 14

[Florian Weimer]

* Eric Gallager via Gcc:

> I support this plan for using -Werror= and having it be split based on
> whether -std= is set to a strict ANSI option or a GNU option; is there
> a way to do that in the optfiles, or would it have to be handled at
> the specs level instead?

This isn't going to work well because -std=c17 (for example) disables
_DEFAULT_SOURCE for glibc, so lots of declarations go missing from the
glibc header files.  I don't see why these things should be tied
together because as I see it, this is not about standards conformance
(warnings make GCC conforming already), but programmer convenience.

Thanks,
Florian