[tz] checklinks.awk validation too strict

[Stephen Colebourne]

Hi,
The latest version of checklinks.awk includes the following check:

    if (backcheck && FILENAME != backcheck && $3 != "GMT") {
      printf "%s: Link should be in '%s'\n", $3, backcheck
      status = 1
    }

The effect of the check is to insist that all Links are in the
backward file. I understand this makes sense for TZDB's default
dataset, as it now contains no Links in the main set of data files. In
my opinion however it is perfectly valid to have a Link in the main
set of files. This has been long-standing practice for TZDB. We know
that some downstream consumers edit the source files before compiling
them, so it seems highly likely that someone will still be wanting to
have a Link in one of the main files.

 (And no, I don't think a command line option to turn it off is the
right solution. If the check needs retaining there should be a
"strict" mode only used by the default TZDB dataset)

In summary, I think that this check should be removed since it is
complaining about perfectly valid input files.

thanks
Stephen