docs/gen-html-index: Make HTML::TreeBuilder::XPath optional again

7782db9260 "docs/gen-html-index: Extract titles from HTML documents"
requires HTML::TreeBuilder::XPath.

This is sadly not as widely available as I had hoped.  Work around
this problem by making the use of this module optional: instead of
`use'ing at the toplevel, we `require' it in the eval.  If it's not
present, then the title is simply not extracted and the filename is
used as before, which is tolerable.

Also add some debugging.

Reported-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Tested-by: Doug Goldstein <cardoe@cardoe.com>

docs/gen-html-index

@@ -10,7 +10,6 @@ use warnings;
 use Getopt::Long;
 use IO::File;
 use File::Basename;
-use HTML::TreeBuilder::XPath;
 
 Getopt::Long::Configure('bundling');
 
@@ -21,8 +20,10 @@ our @dirs;
 our %index;
 
 our $outdir;
+our $debug;
 
-GetOptions("i=s" => sub { read_index(@_);} )
+GetOptions("i=s" => sub { read_index(@_);},
+           "D" => \$debug)
     or die;
 
 ($outdir,@docs) = @ARGV;
@@ -68,6 +69,7 @@ sub make_linktext ($) {
 
     my $from_html;
     eval {
+        require HTML::TreeBuilder::XPath;
         my $tree = new HTML::TreeBuilder::XPath;
         my $f = "$outdir/$l.html";
         open F, '<', $f or die "$l $f $!";
@@ -75,6 +77,7 @@ sub make_linktext ($) {
         close F;
         $from_html = $tree->findvalue("/html/head/title");
     };
+    print "$l: get title: $@" if $@ && $debug;
     return $from_html if $from_html;
 
     return basename($l);