Provide pre-built icons in the source tarball.

Provide pre-built icons in the source tarball.

This reinstates the feature of the previous build system, that the C
icon files for the GTK puzzles were included in the source tarball, so
that users building from that instead of from the raw git repo would
not need to run the fiddly piece of build that regenerates them.

Running that fiddly piece of build is much easier in the CMake world
(because it's integrated with the main makefile), but it has a build
dependency on ImageMagick which is easily avoided.

The makefile will still build the icons if it _can_. But in the case
where it can't, it will use pre-built icon source files if they're
available, and only fall back to no-icon.c if it can't even do that.
(So a user checking out from git and building without ImageMagick
present will still be able to build _something_ playable.)