gtk/: Fix build without PangoFT2 and HarfBuzz

gtk/: Fix build without PangoFT2 and HarfBuzz

On Windows HarfBuzz and PangoFT2 are optional, so we need to ensure that
we only build the bits that require HarfBuzz and PangoFT2 when needed.
We may need to see later whether we can get the needed functionality in
gtkfontchooserwidget.c with the Windows APIs without the need of
HarfBuzz nor PangoFT2 (and starting programs on Windows using FontConfig
is very slow).