Log Message:
Upgrade games/xboard to v4.9.1

Optionally build with gtk2 support instead of xaw. Upstream defaults
to gtk, but we are sticking with xaw for now.

XXX the gnuchess engine will still quit after the second move, see
    <https://lists.gnu.org/archive/html/bug-gnu-chess/2017-08/threads.html>

From upstream's changelog:

** Version 4.9.1 **

   bugfix release for 4.9.0

   * Fix crash when logging out from ICS
   * Defer book faking input move until ping balance
   * Fix setting up btm positions with 'edit'
   * Fix bare King adjudication
   * Fix check testing in games without King
   * Fix disambiguating Pawn moves in Xiangqi
   * Fix King leaving Palace in Xiangqi
   * Fix compile error Xaw build

** Version 4.9.0 **

   (see http://www.gnu.org/software/xboard/whats_new/4.9.0/ )

  * New features in this release
    * Running EPD test suites
    * Getting instant help (XB)
    * Editing castling rights in a position
    * Use mouse wheel to set up position (XB)
    * Allow engine to pop up its settings dialog
    * Specifying ultra-fast games (XB)
    * Installing board themes from packages (XB)
    * Using a single-button mouse
    * A special directory for start positions
    * Allow terminal-less ICS play (XB)
    * Sound warnings in analysis mode
    * Changing fonts through the GUI (XB)
    * Automatic adapting of fonts to board size (XB)

  * New variant support
    * New piece types
    * Allowing more than 26 piece types
    * More flexible piece promotion
    * Restricting promotion choice
    * New configurable move types
    * Inscribing pieces with text
    * Setting rules from the command line
    * Allow inaccessible squares
    * Load Shogi games in .kif format