[cesarb]

I just tried (with gcc 9.2.1), and got no warning. As far as I know, these _s versions aren't that good, and often aren't available at all; see http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1967.htm which says among other things

"[...] none of the popular Open Source distribution such as BSD or Linux has chosen to make either available to their users. At least one (GNU C Library) has repeatedly rejected proposals for inclusion [...]"

"[...] As a result of the numerous deviations from the specification the Microsoft implementation cannot be considered conforming or portable."

[iamnotacrook]

"often aren't available at all"

It's in C11. Perhaps Red Hat use non-compliant compilers, or very, very old ones?

[cesarb]

It's an optional part of C11, which means that even a fully compliant compiler does not have to implement them.