Stop clang from complaining about enum usage.

This commit stops clang from complaining about:

>  demux.c:2033:23: warning: comparison of constant -1 with expression of type 'enum print_type_t' is always true [-Wtautological-constant-out-of-range-compare]
>    if ( i_print_type != -1 )
>         ~~~~~~~~~~~~ ^  ~~
Fix compilation under OSX.

OS X (and FreeBSD) have their own "strict ip" definition. It is
binary compatible with Linux "struct iphdr".