Substantial reworking of Solo so that it implements both Sudoku-X

(require both main diagonals to have one of every digit in addition
to all the usual constraints) and Jigsaw Sudoku (replace the array
of rectangular sub-blocks with the sub-blocks being random
polyominoes). To implement the latter, I've moved my `divvy.c'
library routine out of the `unfinished' subdirectory.

Jigsaw mode is currently an undocumented feature: you enable it by
setting the rows parameter to 1 (and the columns parameter to your
desired grid size, which unlike normal Sudoku can be anything you
like including a prime number). The reason it's undocumented is
because generation times are not yet reliably short: sometimes
generating a jigsaw-type puzzle can hang for hours and still get
nowhere. (The algorithm should terminate in principle, but not in
any time you're prepared to wait.) I _think_ I know how to solve
this, but have yet to try it. Until then, jigsaw mode will remain a
hidden feature.

Printing of X-type puzzles is also substandard at present, because
the current print-colour API replaces the desired light shading of
the X-cells with heavy diagonal hatching. I plan to adjust the API
imminently to address this.

[originally from svn r7974]

solo.R

@@ -1,13 +1,15 @@
 # -*- makefile -*-
 
-solo     : [X] GTK COMMON solo solo-icon|no-icon
+SOLO     = solo divvy dsf
 
-solo     : [G] WINDOWS COMMON solo solo.res|noicon.res
+solo     : [X] GTK COMMON SOLO solo-icon|no-icon
 
-solosolver :    [U] solo[STANDALONE_SOLVER] STANDALONE
-solosolver :    [C] solo[STANDALONE_SOLVER] STANDALONE
+solo     : [G] WINDOWS COMMON SOLO solo.res|noicon.res
 
-ALL += solo
+solosolver :    [U] solo[STANDALONE_SOLVER] dsf STANDALONE
+solosolver :    [C] solo[STANDALONE_SOLVER] dsf STANDALONE
+
+ALL += SOLO
 
 !begin gtk
 GAMES += solo