This patch fixes the compilation of netlink.c and interface.c on those
systems missing either libnl or that have an older linux/if_link.h
include file not supporting macvtap or VF_PORTS.

WITH_MACVTAP is '1' if newer include files were detected, '0' otherwise.

IFLA_PORT_MAX is defined in linux/if_link.h if yet more functionality is
supported.