This header is more or less the same across libc implementations.

`musl` uses its `_Noreturn` macro to detect which `noreturn`
should be used: C11 or GCC's extension as a fallback.

`glibc` simply defines `noreturn` as `_Noreturn`.

This implementation is based off of `musl`'s.

`_Noreturn` is deprecated as of C23.