Use RFC3542 for IPv6 on macOS.

interface.c

@@ -20,6 +20,7 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
+#define __APPLE_USE_RFC_3542
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>

net.c

@@ -21,6 +21,7 @@ THE SOFTWARE.
 */
 
 #define _GNU_SOURCE
+#define __APPLE_USE_RFC_3542
 #include <unistd.h>
 #include <stdio.h>
 #include <fcntl.h>