broker: properly check for getaddrinfo_a()

getaddrinfo_a( is protected by _GNU_SOURCE.

This is properly accounted for in the source code, in lib/net_mosq.c,
but the check in the CmakeList is missing that define, which causes
the check to actually fail.

Add that define when checking; this fixes builds on various C libraries,
like uClibc-ng, musl, or newer/older glibc.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>

src/CMakeLists.txt

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