Various Perl scripts we use to generate files were in the habit of
printing things like "generated by $0" into their output files.
That looks like a fine idea at first glance, but it results in
non-reproducible output, because in VPATH builds $0 won't be just
the name of the script file, but a full path for it.  We'd prefer
that you get identical results whether using VPATH or not, so this
is a bad thing.

Some of these places also printed their input file name(s), causing
an additional hazard of the same type.

Hence, establish a policy that thou shalt not print $0, nor input file
pathnames, into output files (they're still allowed in error messages,
though).  Instead just write the script name verbatim.  While we are at
it, we can make these annotations more useful by giving the script's
full relative path name within the PG source tree, eg instead of
Gen_fmgrtab.pl let's print src/backend/utils/Gen_fmgrtab.pl.

Not all of the changes made here actually affect any files shipped
in finished tarballs today, but it seems best to apply the policy
everyplace so that nobody copies unsafe code into places where it
could matter.

Christoph Berg and Tom Lane

Discussion: https://postgr.es/m/20171215102223.GB31812@msg.df7cb.de