Use gnu17 for clang on MacOS

[nmburgan]

Newer Xcode (16+) defaults clang to -std=gnu23 and C23 changes semantics in ways that can turn previously valid C into undefined behavior, which can cause segfaults.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)