On Linux, certain GNU extensions are available if the _GNU_SOURCE feature test macro is set.
For instance, in order to use POSIX_SPAWN_USEVFORK in posix_spawn this feature test macro must be set, see posix_spawn(3). However, there does not appear to be a libc::POSIX_SPAWN_USEVFORK

Is it correct that libc's binding were created without _GNU_SOURCE and if so, would it be meaningful to create an option in libc to enable these GNU extensions on platforms that support them?