Migrate to a CMake-based build system.

This completely removes the old system of mkfiles.pl + Recipe + .R
files that I used to manage the various per-platform makefiles and
other build scripts in this code base. In its place is a
CMakeLists.txt setup, which is still able to compile for Linux,
Windows, MacOS, NestedVM and Emscripten.

The main reason for doing this is because mkfiles.pl was a horrible
pile of unmaintainable cruft. It was hard to keep up to date (e.g.
didn't reliably support the latest Visual Studio project files); it
was so specific to me that nobody else could maintain it (or was even
interested in trying, and who can blame them?), and it wasn't even
easy to _use_ if you weren't me. And it didn't even produce very good
makefiles.

In fact I've been wanting to hurl mkfiles.pl in the bin for years, but
was blocked by CMake not quite being able to support my clang-cl based
system for cross-compiling for Windows on Linux. But CMake 3.20 was
released this month and fixes the last bug in that area (it had to do
with preprocessing of .rc files), so now I'm unblocked!

CMake is not perfect, but it's better at mkfiles.pl's job than
mkfiles.pl was, and it has the great advantage that lots of other
people already know about it.

Other advantages of the CMake system:

 - Easier to build with. At least for the big three platforms, it's
   possible to write down a list of build commands that's actually the
   same everywhere ("cmake ." followed by "cmake --build ."). There's
   endless scope for making your end-user cmake commands more fancy
   than that, for various advantages, but very few people _have_ to.

 - Less effort required to add a new puzzle. You just add a puzzle()
   statement to the top-level CMakeLists.txt, instead of needing to
   remember eight separate fiddly things to put in the .R file. (Look
   at the reduction in CHECKLST.txt!)

 - The 'unfinished' subdirectory is now _built_ unconditionally, even
   if the things in it don't go into the 'make install' target. So
   they won't bit-rot in future.

 - Unix build: unified the old icons makefile with the main build, so
   that each puzzle builds without an icon, runs to build its icon,
   then relinks with it.

 - Windows build: far easier to switch back and forth between debug
   and release than with the old makefiles.

 - MacOS build: CMake has its own .dmg generator, which is surely
   better thought out than my ten-line bodge.

 - net reduction in the number of lines of code in the code base. In
   fact, that's still true _even_ if you don't count the deletion of
   mkfiles.pl itself - that script didn't even have the virtue of
   allowing everything else to be done exceptionally concisely.

CHECKLST.txt

@@ -6,20 +6,9 @@ Things to remember when adding a new puzzle
 
 Write the source file for the new puzzle (duhh).
 
-Create a .R file for it which:
- - defines a <puzzle>_EXTRA symbol for it if it requires auxiliary
-   object files (make sure that symbol doesn't contain the icon)
- - adds it to the `ALL' definition, to ensure it is compiled into
-   the OS X binary
- - adds it as a GTK build target, with the optional GTK icon
- - adds it as a Windows build target, with the optional resource
-   file
- - adds auxiliary solver binaries if any
- - adds it to $(GAMES) in both the automake and GTK makefiles, for
-   `make install'
- - adds it to list.c for the OS X binary
- - adds it to gamedesc.txt, with its Windows executable name, display
-   name, and slightly longer description.
+Write a puzzle() statement in CMakeLists.txt containing all the
+necessary metadata. Optionally also a solver() statement, and any
+cliprogram() statements for extra auxiliary tools.
 
 If the puzzle is by a new author, modify the copyright notice in
 LICENCE and in puzzles.but. (Also in index.html, but that's listed
@@ -47,15 +36,10 @@ html/<puzzlename>.html .
 
 Make a screenshot:
  - create an appropriate save file in `icons'
- - add the puzzle name to icons/Makefile
- - set up a REDO property in icons/Makefile if the screenshot wants
-   to display a move halfway through an animation
- - set up a CROP property in icons/Makefile if the icon wants to be
-   a sub-rectangle of the whole screenshot
-
-Don't forget to `git add' the new source file, the new .R file and the
-save file in `icons', the new .html file, and any other new files that
-might have been involved.
+ - define <puzzlename>_redo in icons/icons.cmake if the screenshot
+   wants to display a move halfway through an animation
+ - define <puzzlename>_crop in icons/icons.cmake if the icon wants to
+   be a sub-rectangle of the whole screenshot
 
 Check in!