cc -O2 -pthread  -Wall -Wextra -Werror -Wno-error=deprecated -Wno-error=deprecated-declarations unistd/_SC_IPV6.c -o unistd/_SC_IPV6 -D_POSIX_C_SOURCE=202405L  -lm -lpthread -lrt  -lintl -lcrypt
unistd/_SC_IPV6.c:2:17: error: '_SC_IPV6' undeclared here (not in a function)
    2 | int const foo = _SC_IPV6;
      |                 ^~~~~~~~