Buildkernel broken at net/gif

[walt <wa1ter@xxxxxxxxxxxxx>]

Well, okay, not completely broken:  I usually don't include INET6
in my custom kernel, and today I got this error:

/usr/src/sys/net/gif/../../netinet6/in6_gif.c:80: `in6_gif_input' undeclared
here (not in a function)

The 'undeclared' function appears to be defined in
/usr/include/netinet6/in6_gif.h

This error disappeared when I included INET6 in my kernel config.

I want to understand how these failures occur -- so someday I can fix them --
I'd be grateful if someone could explain to me why this particular failure
happened.

Thanks!