compat: Substitute more dependable define

compat: Substitute more dependable define

The compat layer ip_tunnel_get_stats64 function was checking for the
Linux kernel version to determine if the return was void or a pointer.
This is not very reliable and caused compile warnings on SLES 12 SP3.
In acinclude.m4 create a more reliable method of determining when to
use a void return vs. a pointer return.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>