doc: use bold style for man page references

It's more common to use bold style than not,
for references to other man pages.
Ideally each man page renderer would highlight references,
but currently some rely on styles in the page itself.

* man/help2man: Implement a --bold-refs option that
will mark up references like "name(1)" with bold
style around the "name" component.
* man/local.mk: Pass --bold-refs to our help2man unless disabled.
* configure.ac: Add a --disable-bold-man-page-references option.
Addresses https://bugs.gnu.org/53977

man/help2man

@@ -78,6 +78,7 @@ Usage: %s [OPTION]... EXECUTABLE
  -p, --info-page=TEXT    name of Texinfo manual
  -N, --no-info           suppress pointer to Texinfo manual
  -l, --libtool           exclude the `lt-' from the program name
+ -b, --bold-refs         apply bold style to references
      --help              print this help, then exit
      --version           print version number, then exit
 
@@ -99,7 +100,7 @@ my $help_option = '--help';
 my $version_option = '--version';
 my $discard_stderr = 1;
 my ($opt_name, @opt_include, $opt_output, $opt_info, $opt_no_info, $opt_libtool,
-    $version_text);
+    $opt_bold_refs, $version_text);
 
 my %opt_def = (
     'n|name=s'		 => \$opt_name,
@@ -113,6 +114,7 @@ my %opt_def = (
     'p|info-page=s'	 => \$opt_info,
     'N|no-info'		 => \$opt_no_info,
     'l|libtool'		 => \$opt_libtool,
+    'b|bold-refs'	 => \$opt_bold_refs,
     'help'		 => sub { print $help_info; exit },
     'version'		 => sub { print $version_info; exit },
     'h|help-option=s'	 => \$help_option,
@@ -714,6 +716,13 @@ for my $sect (@pre, (grep !$filter{$_}, @sections), @post)
 
 	for ($include{$sect})
 	{
+            # Add bold style around referenced pages.
+            if ($opt_bold_refs)
+            {
+                # This will ignore entries already marked up (with \)
+                s/(^|\s|,)([\[\w\x83]+)\(([1-9][[:lower:]]?)\)/$1\\fB$2\\fP($3)/g;
+            }
+
 	    # Replace leading dot, apostrophe, backslash and hyphen
 	    # tokens.
 	    s/\x80/\\&./g;

man/local.mk

@@ -24,7 +24,10 @@ run_help2man = $(SHELL) $(srcdir)/man/dummy-man
 else
 ## Graceful degradation for systems lacking perl.
 if HAVE_PERL
-run_help2man = $(PERL) -- $(srcdir)/man/help2man
+if BOLD_MAN_REFS
+help2man_OPTS=--bold-refs
+endif
+run_help2man = $(PERL) -- $(srcdir)/man/help2man $(help2man_OPTS)
 else
 run_help2man = $(SHELL) $(srcdir)/man/dummy-man
 endif