Don't use missingIncludes unless custom syntax defs have been given.

This avoids a huge performance sink that comes from evaluating
all the elements of the default syntax map.

Better just to have run-time errors for missing includes?

See #4226.

src/Text/Pandoc/App.hs

@@ -268,7 +268,8 @@ convertWithOpts opts = do
   syntaxMap <- foldM addSyntaxMap defaultSyntaxMap
                      (optSyntaxDefinitions opts)
 
-  case missingIncludes (M.elems syntaxMap) of
+  unless (null (optSyntaxDefinitions opts)) $
+    case missingIncludes (M.elems syntaxMap) of
        [] -> return ()
        xs -> E.throwIO $ PandocSyntaxMapError $
                 "Missing syntax definitions:\n" ++