Newer texlive distributions require \usepackage{float} for
the [H] figure layout directive. We use this to make sure
the header layout figures are at the top of their respective
sections for conventional reference like in an RFC, so requiring
the additional package is safer than falling back to standard
latex [h] or [!h].

Note that [H] from the float package is incompatible with
fallback specifiers.

The configure script will automatically check for the new package.