Set _GNU_SOURCE in asyncsendto.c
Created by: ffontaine
Add #define on _GNU_SOURCE at the very beginning of asyncsendto.c file as this file uses in6_pktinfo which is protected by __USE_GNU in libc/inet/netinet/in.h. Currently, this flag is set by the Makefile in CFLAGS however CFLAGS could be overwritten by the build system.
Signed-off-by: Fabrice Fontaine fabrice.fontaine@orange.com