[CMake] Clear cache on upgrade?

[John Drescher]

> A good rule of thumb is to try just upgrading CMake and running it on
> existing build trees. It's obviously quicker than a re-configure from
> scratch.
>
> But then, before complaining about something not working, try it in a fresh
> build tree first, then if it's still wrong, complain. :-)
>
> It's rare, although it does happen sometimes, that we make a change in CMake
> itself that invalidates something that's in an existing cache.
>

Fully agreed.. It has been quite rare that a CMake upgrade has caused
a problem or a need to re configure.

John