BLD: Bump our C standard to 11

[lithomas1]

We're getting Cython compilation errors with Cython < 3 and C99, on the wheel builders.

NOTE: Cython 3 seems to handle this OK.

Let's see if C11 fixes it since we can't go directly to Cython 3 yet.

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[WillAyd]

FWIW I wouldn't mind even jumping to C17. I don't think people compile against pandas so I don't think we need to worry about supporting old standards.

I'd really love to jump to C23 as soon as all compilers support it

[lithomas1]

FWIW I wouldn't mind even jumping to C17. I don't think people compile against pandas so I don't think we need to worry about supporting old standards.

I'd really love to jump to C23 as soon as all compilers support it

Is there anything we need from C17?

IIUC, the C standard doesn't really change that much anymore (at least not a lot compared to C++), and I'd prefer to only bump if we need something from a newer C standard.

[WillAyd]

C17 doesn't have any new features. Just bug fixes for C11

[lithomas1]

@WillAyd OK to merge here?

I'm a little worried about requiring recent compilers for C17 and above, so maybe we can discuss that in a separate issue?

[WillAyd]

Ok sure merge away. C17 is just bug fixes so I'm not sure which compilers would support one not the other today. But in either case this is an upgrade over C99

[WillAyd]

Thanks @lithomas1 good to get this updated