avoid running test if xml::twig is not installed

t/map.t

@@ -3,9 +3,16 @@ package IkiWiki;
 
 use warnings;
 use strict;
-use XML::Twig;
 use Test::More;
 
+BEGIN {
+        unless (eval { require XML::Twig }) {
+                eval q{
+                        use Test::More skip_all => "XML::Twig is not available"
+                }
+        }
+}
+
 BEGIN { use_ok("IkiWiki"); }
 BEGIN { use_ok("IkiWiki::Render"); }
 BEGIN { use_ok("IkiWiki::Plugin::map"); }