BUILD/MINOR: listener: remove a glibc warning on accept4()

The accept4() Linux syscall requires _GNU_SOURCE on ix86, otherwise
it emits a warning. On other archs including x86_64, this problem
doesn't happen. Thanks to Charles Carter from Sigma Software for
reporting this.

src/listener.c

@@ -10,6 +10,7 @@
  *
  */
 
+#define _GNU_SOURCE
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>