Actually I just need it in Sessions.cc (for the switch
on the ICMP type)
so I will use the #undef solution : it's less ugly than keeping a
modified version of icmp6.h.
I think you misunderstood me. If you just need it in Sessions.cc, then
just include it in Sessions.cc rather than in net_util.h (and include it
after DNS-binpac.h, with a comment, if that's the only conflict). A little
better than using an #undef.