add SOURCELINK_SUFFIX for compatibility with Sphinx 1.5

With Sphinx 1.5, this is needed by searchtools.js to display the source
snippets (see sphinx-doc/sphinx#2454).

With earlier Sphinx versions, this is a no-op because the undefined variable
will evaluate to an empty string.

docs/_isso/layout.html

@@ -10,7 +10,8 @@
         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 %}