Blindly try to fix MSVC build's use of genbki.pl and Gen_fmgrtab.pl.

Blindly try to fix MSVC build's use of genbki.pl and Gen_fmgrtab.pl.

We need to use a stamp file to record the runs of these scripts, as
is done on the Unix side.  I think I got it right, but can't test.

While at it, extend this handmade dependency logic to also check the
generating script files, as the makefiles do.

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