Addendum to the fix to issue #242.

The previous fix resulted in bird tracks being included in
both html and html+lhs renderings of literate haskell sections
when pandoc was compiled without highlighting support. This change make
pandoc without highlighting behave like pandoc with highlighting: the
bird tracks are used only if html+lhs output is specified.

src/Text/Pandoc/Writers/HTML.hs

@@ -285,7 +285,7 @@ blockToHtml opts (CodeBlock (id',classes,keyvals) rawCode) = do
                         attrs = [theclass (unwords classes') | not (null classes')] ++
                                 [prefixedId opts id' | not (null id')] ++
                                 map (\(x,y) -> strAttr x y) keyvals
-                        addBird = if "literate" `elem` classes
+                        addBird = if "literate" `elem` classes'
                                      then unlines . map ("> " ++) . lines
                                      else unlines . lines
                     in  return $ pre ! attrs $ thecode <<