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.

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