spell: allow overriding dictionaries locations

Works for aspell and myspell (hunspell) when using enchant.

src/plugins/spell/spell-command.c

@@ -157,6 +157,9 @@ spell_command_speller_list_dicts (void)
                                NULL);
 #else
     config = new_aspell_config ();
+#ifdef ASPELL_DICT_DIR
+    aspell_config_replace (config, "dict-dir", ASPELL_DICT_DIR);
+#endif
     list = get_aspell_dict_info_list (config);
     elements = aspell_dict_info_list_elements (list);