The current setup assumes that commands for lz4, zstd and gzip always
exist by default if not enforced by a user's environment.  However,
vcpkg, as one example, installs libraries but no binaries, so this
default setup to assume that a command should always be present would
cause failures.  This commit improves the detection of such external
commands as follows:
* If a ENV value is available, trust the environment/user and use it.
* If a ENV value is not available, check its execution by looking in the
current PATH, by launching a simple "$command --version" (that should be
portable enough).
** On execution failure, ignore ENV{command}.
** On execution success, set ENV{command} = "$command".

Note that this new rule applies to gzip, lz4 and zstd but not tar that
we assume will always exist.  Those commands are set up in the
environment only when using bincheck and taptest.  The CI includes all
those commands and I have checked that their setup is correct there.  I
have also tested this change in a MSVC environment where we have none of
those commands.

While on it, remove the references to lz4 from the documentation and
vcregress.pl in ~v13.  --with-lz4 has been added in v14~ so there is no
point to have this information in these older branches.

Reported-by: Andrew Dunstan
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/14402151-376b-a57a-6d0c-10ad12608e12@dunslane.net
Backpatch-through: 10