wesnoth: only compile tiny GUI when screen is smaller than 800x480(smallgui)

wesnoth: only compile tiny GUI when screen is smaller than 800x480(smallgui)

Wesnoth has a compile time option for making it work on smaller screen
  The minimum is 400x300,else you would have to modify
  /usr/share/wesnoth/data/themes/default.cfg to support your resolution.
  Basically it uses imagemagick-native to shrink the graphics,which leads
  to some problems like for instance:
   *Only a flag can be seen instead of a full village
   *The forest look like grass