Build failure of version 2.5.0 with kernel < 4.7.0

Version 2.5.0 raises the following build failure with kernel < 4.7.0:

sys-linux.c: In function 'get_ppp_stats_rtnetlink':
sys-linux.c:1784:29: error: field 'ifsm' has incomplete type
         struct if_stats_msg ifsm;
                             ^

This build failure is raised because if_stats_msg is unconditionally used since 81ad945. However, if_stats_msg is only available since kernel 4.7.0 and torvalds/linux@10c9ead

I would advise to add this dependency somewhere in the README and perhaps also drop those unneeded comments/defines such as

ppp/pppd/sys-linux.c

Line 135 in f5aa69b

/* Attempt at retaining compile-support with older than 4.7 kernels, or kernels

@paulusmack, @enaess: Have you seen this ticket?

@jkroonza Looks like commit 81ad945 didn't really manage to achieve backward compatibility - could you comment?

That's because I was being a moron thinking only in terms of runtime and not in terms of compile time.

@ffontaine could you please test and confirm PR #452? I don't have a <4.7.0 kernel lying around to test against, but I can't see how it won't fix the problem.