Issue 5593: Optionally use "tidy" to check generated HTML

https://sourceforge.net/p/testlilyissues/issues/5593/

1: use "false" for missing programs

STEPMAKE_OPTIONAL_REQUIRED sets "false" for a missing program, whether
required or optional.

I removed a comment suggesting to abort immediately when a required
program is missing; that would be less helpful than the current behavior
of reporting all missing programs when the script is done.

Thanks to Jonas Hahnfeld <hahnjo@hahnjo.de> for correcting a
misunderstanding I had about this code, leading to this improvement.

2: optionally use "tidy" to check generated HTML

I found the program "tidy" to be somewhat useful for catching problems
like empty or unclosed tags in the HTML generated by output-distance,
so I added it to the build as an option.  http://validator.w3.org
caught a few more problems, but tidy is fast and I didn't see it raise
any incorrect objections, so it seems beneficial to use it when it is
available.

[Dan Eble, 2019-11-05 14:42:31]

no change

[Dan Eble, 2019-11-05 14:48:30]

no change

[Dan Eble, 2019-11-05 14:52:19]

no change

[Dan Eble, 2019-11-05 14:56:07]

no change

[hahnjo, 2019-11-05 16:29:28]

I even like this better than my original version, always having false for a
missing program sounds like a good idea.