Set SOURCE_DATE_EPOCH to make the build reproducible.
    
    The timestamp of generated documentation changes from build to build.
    This makes the builds not reproducible, because the timestamp changes.
    RDoc has allows to override the timestamps via SOURCE_DATE_EPOCH
    environment variable, which was intdouced by following PRs:
    
    https://github.com/ruby/rdoc/pull/569/
    https://github.com/ruby/rdoc/pull/570
    
    This is related to rhbz#1719647. Nevertheless, that specific issue is
    for plaid ruby- packages, so these macros cannot help with that specific
    case.