Build WITH_ADNS fails on Debian Forky

[narc-Ontakac2]

This is caused by a missing #define _GNU_SOURCE.

[narc-Ontakac2]

This is fixed by

--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -184,6 +184,7 @@
 if(WITH_ADNS)
 	cmake_push_check_state()
 	set(ANL_CODE "
+        #define _GNU_SOURCE
 		#include <stdlib.h>
 		#include <netdb.h>
 		int main(){

As in #3548 , I hope it is OK to do this without a pull request.