Need a CI check with `--std=c99` set

[aitap]

As mentioned by @MichaelChirico in #7517 (comment), we need a check that hides by default all the non-standard extensions that the code may be using (because they may be hidden by default on e.g. FreeBSD). Then our code will not pass the check without explicitly asking for them (e.g. by #define-ing _POSIX_C_SOURCE). We have hit this problem before when a change that used a C23 feature passed CI but failed with cc() (which now uses -std=c11).