I'm sick of repeatedly adding and removing local changes to Recipe

when testing a new game, so here's a new architecture for the Recipe
file. mkfiles.pl now supports several new features:

 - an `!include' directive, which accepts wildcards
 - += to append to an existing object group definition
 - the ability to divert output to an arbitrary file.

So now each puzzle has a `.R' file containing a fragment of Recipe
code describing that puzzle, and the central Recipe does `!include
*.R' to construct the Makefiles. That way, I can keep as many
experimental half-finished puzzles lying around my working directory
as I like, and I won't have to keep reverting Recipe when I check in
any other changes.

As part of this change, list.c is no longer a version-controlled
file; it's now constructed by mkfiles.pl, so that it too can take
advantage of this mechanism.

[originally from svn r6781]

tents.R

@@ -0,0 +1,20 @@
+# -*- makefile -*-
+
+TENTS    = tents maxflow
+
+tents    : [X] GTK COMMON TENTS
+
+tents    : [G] WINDOWS COMMON TENTS
+
+ALL += TENTS
+
+tentssolver :   [U] tents[STANDALONE_SOLVER] maxflow STANDALONE
+tentssolver :   [C] tents[STANDALONE_SOLVER] maxflow STANDALONE
+
+!begin gtk
+GAMES += tents
+!end
+
+!begin >list.c
+    A(tents) \
+!end