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

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]