This is a fix for the QA warnings that result from using asprintf()
without defining it. That happens because asprintf() is a GNU
extension, but somehow gets used before _GNU_SOURCE is defined.

Upstream-Bug: https://github.com/NagiosEnterprises/ndoutils/issues/43

--- a/include/config.h.in
+++ b/include/config.h.in
@@ -9,6 +9,7 @@
 #ifndef _CONFIG_H
 #define _CONFIG_H
 
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>