Release needs to update PIVOT.h

[Hal Murray]

Where is your checklist for a release?

I've added PIVOT.h at the top level.  It's only content is a #define for 
RELEASE_DATE.

That's the magic constant that's needed for sanity checking and pivoting on 
date conversions because using the build date would break repeatable builds.

It should be updated shortly before the release.  The release announcement 
would be a good time.  The catch is that changing the pivot date might break 
some of the tests.

-- 
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