Set SOURCE_DATE_EPOCH and TZ to improve build reproducibility
    
    The documentation tools respect these variables when generating dates in
    the man and html docs.  This is a small step toward making the package
    builds reproducible.
    
    An alternate method to set SOURCE_DATE_EPOCH would be to set the rpm
    %source_date_epoch_from_changelog macro.  Using the version file from
    the tarball is a little nicer as the date is printed in the man pages.
    We'd still need to set TZ anyway, as the html documentation sets the
    'last updated' footer entry based on the timestamp of the corresponding
    txt file.
    
    [Note: It is possible to avoid the 'last updated' footer entirely by
    setting the asciidoc footer-style attribute to none.  This would need to
    be done via sed or a patch, as there's not currently a way to set this
    in config.mak -- but perhaps there should be.]
    
    Reference: https://reproducible-builds.org/specs/source-date-epoch/