[CMake] ExternalProject danger- source directory is deleted

[Pat Marion]

When using ExternalProject, sometimes the build rules decide that an
existing source directory needs to be deleted and re{downloaded,cloned}.
In my experience, this is pretty dangerous and I have lost work due to it.
Would it be possible to throw an error and ask the user to perform the
delete, or pass a --force argument?  Is it the case that dashboards rely on
the ability to do a hard-delete of a source directory during intermediate
builds?

Pat
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