If a particular output file already exists with the contents it should
have, leave it alone, so that its mod timestamp is not advanced.

In builds using --enable-depend, this can avoid the need to recompile .c
files whose included files didn't actually change.  It's not clear whether
it saves much of anything for users of ccache; but the cost of doing the
file comparisons seems to be negligible, so we might as well do it.

For developers using the MSVC toolchain, this will create a regression:
msvc/Solution.pm will sometimes run genbki.pl or Gen_fmgrtab.pl
unnecessarily.  I'll look into fixing that separately.

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