fix if.h problems when compiling with newer libnl3
    
    When netcf first include support for libnl3, the author of that patch
    (Serge Hallyn, commit 0310cd505) found that
    /usr/include/libnl3/netlink/route/link.h #included <linux/if.h>, which
    conflicts with <net/if.h> (but for our purposes at least defines all the
    same things). So he put the #include <net/if.h> in dutil_linux.c inside
    an #ifndef HAVE_LIBNL3.
    
    That worked fine until libnl3 finally removed the offending (and
    unnecessary) #include <linux/if.h> from its link.h. That fix to libnl3
    broke the netcf build.
    
    This patch fixes the build by using autotools magic to attempt
    compiling essentially this short program:
    
      #include <netlink/route/link.h>
      int main()
      {
          struct ifreq ifr;
          return ifr.ifr_ifru.ifru_ivalue = IFF_UP;
      }
    
    If this can compile successfully without any other includes, then
    link.h is already including linux/if.h and we should avoid including
    net/if.h, so we #define AVOID_NET_IF_H. If the compile fails, then we
    do need to include net/if.h, so we *don't* #define
    AVOID_NET_IF_H. Then we enclose dutil_linux.c's #include <net/if.h>
    within #ifndef AVOID_NET_IF_H and we're done.
    
    Although this check would probably be safe on a libnl1 system, we know
    from past experience that we should always include net/if.h in those
    cases anyway, so we don't even try - we only do the configure-time
    check if we're using libnl3.