Avoid .la files getting removed on "make clean"

These are generated during the cmake stage, but cmake thinks they are
generated during the build stage because of our rule to update the
timestamp on them. As a result, it thinks they should be removed when
doing "make clean".

There doesn't seems to be an easy way to tell cmake to leave these
alone. So instead, point cmake to a different marker file when updating
the timestamps. We don't care if that gets removed as it has no valuable
contents.

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