Some IFLA_* symbols can be missing in the host linux/if_link.h,
but as they are enums and not "#defines", check in "configure" if
last known  (IFLA_PROTO_DOWN) is available and if not, disable
management of NETLINK_ROUTE protocol.
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>