doc: update urls in manual and include checklinkx script.

doc: update urls in manual and include checklinkx script.

* doc/automake.texi: update many urls; http -> https,
search.cpan.org -> metacpan.org/pod/distribution, node names, etc.
Remove sourceware.org/cgi-bin/gnatsweb.pl and
miller.emu.id.au/pmiller/books/rmch/ as these are 404
and no good replacement is evident.
s/perl/Perl/ a couple times in text for good measure.
* contrib/checklinkx: new script, a small modification of
W3C checklink <https://validator.w3.org/checklink/docs/checklink.html>
(W3C license, which is free software), starting from version 4.81
installed from CPAN:
https://metacpan.org/pod/distribution/W3C-LinkChecker/bin/checklink.pod
* doc/local.mk (checklinkx): new target to invoke it, with variables.
* Makefile.am (EXTRA_DIST): distribute it.
* NEWS: mention all this.