Regarding your first question, my platform is NonStop(Non-Standard but similar to IBM AIX). Our compilers support C11 standards.
Regarding the second part, yes, my build errors out at pyatomic_std.h with error of stdatomic.h not being present. However, configure goes through with the following message at the end,
configure: Your compiler or platform does have a working C11 stdatomic.h. A future version of Python may require stdatomic.h.
so, if the configure says this, then I assumed that stdatomic.h is optional for compilation of CPython.