Fixed define check for _GNU_SOURCE

m4/ax_recvmmsg.m4

@@ -21,7 +21,9 @@ AC_DEFUN([AX_MSG_WAITFORONE],
     [
       AC_LANG_PROGRAM(
       [
-#define _GNU_SOURCE
+#ifndef _GNU_SOURCE
+  #define _GNU_SOURCE
+#endif
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>