Fix _POSIX_C_SOURCE definition so that getaddrinfo works

@@ -20,7 +20,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#define _POSIX_SOURCE 1
+#define _POSIX_C_SOURCE 200112L
 
 #include <stdlib.h>     // for atoi() and exit()
 #include <errno.h>      // for errno