> I would suggest using my _DARWIN_C_SOURCE implementation
> unconditionally and make similar changes to posix_setgroups, but this
> is probably a subject for a separate issue.

I would propose a different strategy: if _SC_NGROUPS_MAX is defined, use
that to find out how much memory to allocate, otherwise, fall back to
the current max array size. Can you find out whether doing so would also
fix the issue at hand?