mktables: if modify during run, regen tables

mktables: if modify during run, regen tables

This bug has bitten me enough times that I decided to fix it.  It would
only be applicable to someone who is developing mktables, and making
changes to it while the program is running.  Due to the complicated
nature of the dependencies of mktables, it basically functions as its
own make command, knowing when to rebuild or not based on the
modification times of itself and its input and output files.
Previously, it did not account for changes to itself made while running,
so it would think that it wouldn't have to rebuild, even though it did.

I solve this problem by keeping track of the time the process started,
and storing it in mktables.lst.  If something has changed since then,
then the files need to be rebuilt.