Use XFT fonts if available

The silent error `user error (createFontSet)` would break certain
modules (like the prompt) by simply not showing anything.

Pango 1.44 dropped support for FreeType in favor of HarfBuzz, losing
support for traditional BDF/PCF bitmap fonts.  Hence, some distributions
don't ship `xorg-fonts-misc` anymore.

Fixes https://github.com/xmonad/xmonad-contrib/issues/348

XMonad/Actions/ShowText.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE CPP                #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  XMonad.Actions.ShowText
@@ -74,7 +75,11 @@ data ShowTextConfig =
 
 instance Default ShowTextConfig where
   def =
+#ifdef XFT
+    STC { st_font = "xft:monospace-20"
+#else
     STC { st_font = "-misc-fixed-*-*-*-*-20-*-*-*-*-*-*-*"
+#endif
         , st_bg   = "black"
         , st_fg   = "white"
     }