Track catalog & magic prereqs and diff for changes

If a new prerequisite for a catalog (or the compiled magic file) is
added, and it is newer than the last build, then the catalog (or
compiled magic file) will be rebuilt. However, formerly, the file would
not be rebuilt if the added file was older, or if a prerequisite was
removed instead of added, due to limitations in Make.

This commit tracks the list of prerequisites separately, and if it
changes, forces a rebuild of the file regardless of whether the
prerequisites are newer or older than the target.