Possible incompleteness in INPUT-FILES for DEFINE-OP

[Robert Goldman]

Hm.  On a little more thought, it occurs to me that these
version-defining files can't be normal dependencies, because they aren't
loaded like other files, they are included.

Hm.  So they only affect computing whether or not a system definition is
up-to-date -- they aren't proper dependencies.  That's unfortunate,
because it adds a special case.