build: man page generation: degrade gracefully when perl is lacking

Since commit v8.19-118-g57da212, our 'dist-hook' rule tweaked the
distributed Makefile.in to make each man page 'man/foo.1' depend
on the corresponding source 'src/foo.c' rather than the corresponding
program 'src/foo'.  That was done to accommodate systems without
perl, which cannot run help2man to regenerate the manpage after
its corresponding program has been built.

This seems a right and proper graceful degradation, in that the
man pages dependencies are still 100% correct in a git checkout,
while being more lax but "more portable" in a distribution tarball.
Alas, that is not the case in practice, as it turns out the tweaked
Makefile makes the building of man pages unreliable and potentially
incorrect!

In fact, assume that instead of the correct a dependency:

    man/ls.1: src/ls

we have the laxer one:

    man/ls.1: src/ls.c

and think of what happens if a user modifies, say, 'src/ls.c', and then
runs "make -j4" to rebuild everything.  The make process will see that
it has to rebuild the man page 'man/ls.1' (because its prerequisite
'src/ls.c' has changed), but won't see that it has to rebuild 'src/ls'
*before* re-running 'help2man' to generate that man page; so, if
'man/ls.1' is rebuilt before 'src/ls' (which can happen with concurrent
make), our user will get either a build error (if 'src/ls' did non
exist) or, worse, a man page with an up-to-date timestamp but an
out-of-date content.  And what's even worse in all of this is that
this problem will be present also for users who have perl installed:
this is not a "graceful degradation" at all!

In our situation, the best and simplest way to implement a graceful
degradation it to keep the correct dependencies for man pages (that
is, "man/ls.1: src/ls"), and if perl is not present, just generate
dummy man pages reporting that built-time issue and redirecting the
user back to either the info documentation or the '--help' output.

As a consequence of this change, we also stop distributing man pages,
since they will now be generated unconditionally.

* Makefile.am (do-not-require-help2man): Remove.
(dist-hook): Don't depend on it.
* man/local.mk: Remove an obsolete comment.
(EXTRA_DIST): Stop distributing generated man pages.
($(EXTRA_MANS)): This no longer needs to depend on $(all_programs).
(MAINTAINERCLEANFILES): $(ALL_MANS) Do not list it here, and ...
(CLEANFILES): ... list it here, instead.
(.x.1): Instead of warning if perl is missing, but then trying to run
'help2man' unconditionally, simply run ...
(run_help2man): ... the command referenced by this new variable, that
expands to a proper invocation of 'help2man' if perl is present, and
to an invocation of a shell script generating a dummy manpage if it
is not.
(EXTRA_DIST): Distribute that shell script.
* man/dummy-man: New shell script.
* NEWS (Build-related): Mention this.
Fixes coreutils http://bugs.gnu.org/12715.

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