add SOURCELINK_SUFFIX for compatibility with Sphinx 1.5

doc/basic-bootstrap/layout.html

@@ -90,7 +90,8 @@
         VERSION:     '{{ release|e }}',
         COLLAPSE_INDEX: false,
         FILE_SUFFIX: '{{ '' if no_search_suffix else file_suffix }}',
-        HAS_SOURCE:  {{ has_source|lower }}
+        HAS_SOURCE:  {{ has_source|lower }},
+        SOURCELINK_SUFFIX:  '{{ sourcelink_suffix }}'
       };
     </script>
     {%- for scriptfile in script_files %}