[build]: Refactor grohtml runtime-dep check.

Handle missing programs required at runtime by grohtml more gracefully.
Rename and refactor configuration-time logic to be more understandable.

* configure.ac:
* m4/groff.m4: Rename `GROFF_HTML_PROGRAMS` macro to
  `GROFF_CHECK_GROHTML_PROGRAMS`.  Rename shell variable `make_htmldoc`
  to `use_grohtml`.  Set it explicitly to "no" or "yes" instead of null
  or not null.

* configure.ac:
* m4/groff.m4:
* doc/doc.am: Rename `BUILD_HTML` to `USE_GROHTML`.

* configure.ac: Call `GROFF_GROHTML_PROGRAM_NOTICE`.

* doc/doc.am: Bracket definition of `PROCESSEDDOCFILES_HTML` macro and
  `$(PROCESSEDDOCFILES_HTML)` dependency declaration in Automake `if
  USE_GROHTML` conditional.  This prevents an attempt to build an HTML
  version of the "pic.ms" document that is doomed to fail (noisily).

* m4/groff.m4: Refactor grohtml runtime-dependency program check.  Split
  into two macros: one (`GROFF_CHECK_GROHTML_PROGRAMS`) performs the
  check, the other (`GROFF_GROHTML_PROGRAM_NOTICE`) issues a notice at
  the end of the configuration process if at least one program was not
  found.  Rename `html_docnote` shell variable to `grohtml_notice`.
  Tighten wording of notice.  Drop unused `make_install_htmldoc` and
  `make_uninstall_htmldoc` shell variables.

Also make code style more readable.

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