Make these rules look more like the ones associated with genbki.pl,
to wit:

* Use a stamp file to record when we last ran the script, instead of
relying on the timestamps of the individual output files.

* Take the knowledge out of backend/Makefile and put it in utils/Makefile
where it belongs.  I moved down the handling of errcodes.h and probes.h
too, although those continue to be built by separate processes.

In itself, this is just much-needed cleanup with little practical effect.
However, by decoupling these makefile rules from the timestamps of the
generated header files, we open the door to not advancing those timestamps
unnecessarily, which will be taken advantage of by the next commit.

msvc/Solution.pm should be taught to do things similarly, but I'll leave
that for another commit.

Discussion: https://postgr.es/m/16925.1525376229@sss.pgh.pa.us