Only enable extra ligatures with recent Pango versions

Pango's opentype feature selection functions are only available
from version 1.38+, which is still quite new, so ensure it's just
ignored if using an older version.

training/stringrenderer.cpp

@@ -206,12 +206,14 @@ void StringRenderer::SetLayoutProperties() {
     spacing_attr->end_index = static_cast<guint>(-1);
     pango_attr_list_change(attr_list, spacing_attr);
   }
+#if (PANGO_VERSION_MAJOR == 1 && PANGO_VERSION_MINOR >= 38)
   if (add_ligatures_) {
     set_features("liga, clig, dlig, hlig");
     PangoAttribute* feature_attr =
       pango_attr_font_features_new(features_);
     pango_attr_list_change(attr_list, feature_attr);
   }
+#endif
   pango_layout_set_attributes(layout_, attr_list);
   pango_attr_list_unref(attr_list);
   // Adjust line spacing