ports with bootstrapping dependencies

[Taylor R Campbell]

Actually, it occurs to me that it's not really necessary to declare a
dependency -- that it would suffice for the variant to have a command
that checks for the existence of an executable by some name, and to
refuse to continue if this command fails.  There was some recent
discussion about a `ui_fatal' command by which to fail, although I
understand that it was simply a proposal not yet implemented.  Does
this sound plausible, however?