Advertise user-configurable cmake-time config options.

Various cmake variables that I was informally expecting users to set
on the cmake command line (e.g. cmake -DSTRICT=ON, or cmake
-DPUZZLES_GTK_VERSION=2) are now labelled explicitly with the CACHE
tag, and provided with a documentation string indicating what they're
for.

One effect of this is that GUI-like interfaces to your cmake build
directory, such as ccmake or cmake-gui, will show those variables
explicitly to give you a hint that you might want to change them.

Another is that when you do change them, cmake will recognise that it
needs to redo the rest of its configuration. Previously, if you sat in
an existing cmake build directory and did 'cmake -DSTRICT=ON .'
followed by 'cmake -DSTRICT=OFF .', nothing would happen, even though
you obviously meant it to.

cmake/setup.cmake

@@ -1,3 +1,7 @@
+set(PUZZLES_ENABLE_UNFINISHED ""
+  CACHE STRING "List of puzzles in the 'unfinished' subdirectory \
+to build as if official (separated by ';')")
+
 set(build_individual_puzzles TRUE)
 set(build_cli_programs TRUE)
 set(build_icons FALSE)