This patched over this change in the gtk api, without exposing it to the
Haskell API:
https://git.gnome.org/browse/gtk+/commit/?id=68ad33cb89329cf8161dbd1e868097ac1829c16d


Index: gtk3-0.12.5.6/Graphics/UI/Gtk/Multiline/TextTagTable.chs
===================================================================
--- gtk3-0.12.5.6.orig/Graphics/UI/Gtk/Multiline/TextTagTable.chs	2014-10-13 10:29:46.431712334 +0200
+++ gtk3-0.12.5.6/Graphics/UI/Gtk/Multiline/TextTagTable.chs	2014-10-13 10:30:08.227937577 +0200
@@ -84,10 +84,11 @@
 -- an already-added tag.
 --
 textTagTableAdd :: (TextTagTableClass self, TextTagClass tag) => self -> tag -> IO ()
-textTagTableAdd self tag =
+textTagTableAdd self tag = do
   {# call text_tag_table_add #}
     (toTextTagTable self)
     (toTextTag tag)
+  return ()
 
 -- | Remove a tag from the table.
 --
