Avoid unnecessary timestamp bumps on generated-games.h.

If I re-run cmake in a Unix build directory, it unconditionally
rewrites generated-games.h, which causes fuzzpuzz to be rebuilt. This
is a waste of effort in the extremely common case where the rewritten
generated-games.h is identical to the old one.

Now we write the data to a temporary file first, and use cmake's
'configure_file' command to copy that to generated-games.h, because it
so happens that configure_file checks if the two files are identical
and avoids updating the timestamp on the destination file if so.

(This will presumably also be a beneficial change on any other
platform that uses generated_games.h in the build, such as OS X. I
just hadn't noticed until it hit the build I most often re-run in an
existing build directory.)

cmake 3.21 has a more intuitively spelled command I could have used,
called 'file(COPY_FILE src dst ONLY_IF_DIFFERENT)'. But we currently
permit cmake all the way back to 3.5, so I can't use that.

cmake/setup.cmake

@@ -145,13 +145,16 @@ endfunction()
 function(write_generated_games_header)
   set(generated_include_dir ${CMAKE_CURRENT_BINARY_DIR}/include)
   set(generated_include_dir ${generated_include_dir} PARENT_SCOPE)
+  set(header_pre ${generated_include_dir}/generated-games.h.pre)
+  set(header ${generated_include_dir}/generated-games.h)
 
   file(MAKE_DIRECTORY ${generated_include_dir})
-  file(WRITE ${generated_include_dir}/generated-games.h "")
+  file(WRITE ${header_pre} "")
   list(SORT puzzle_names)
   foreach(name ${puzzle_names})
-    file(APPEND ${generated_include_dir}/generated-games.h "GAME(${name})\n")
+    file(APPEND ${header_pre} "GAME(${name})\n")
   endforeach()
+  configure_file(${header_pre} ${header} COPYONLY)
 endfunction()
 
 # This has to be run from the unfinished subdirectory, so that the