Added a --global-module-index option to specify a (possibly relative) URL
to the Global Module Index for a set of documents.  This is used to include
a reference to the global index from the per-document module indexes, so
that it is just a little easier to find.

(Someone suggested this, but I do not remember who.  Please let me know if
it was you -- thanks!)
diff --git a/Doc/perl/l2hinit.perl b/Doc/perl/l2hinit.perl
index 4ffcc58..d3720d9 100644
--- a/Doc/perl/l2hinit.perl
+++ b/Doc/perl/l2hinit.perl
@@ -332,6 +332,16 @@
               . "<tt class='module'>$key</tt>$plat###\n";
     }
     close(MODIDXFILE);
+
+    if ($GLOBAL_MODULE_INDEX) {
+        $prefix = <<MODULE_INDEX_PREFIX;
+
+<p> This index only lists modules documented in this manual.
+  The <em class="citetitle"><a href="$GLOBAL_MODULE_INDEX">Global Module
+     Index</a></em> lists all modules that are documented in this set
+  of manuals.</p>
+MODULE_INDEX_PREFIX
+    }
     if (!$allthesame) {
 	$prefix .= <<PLAT_DISCUSS;