building with boost-1.73.0 requires changes in bind placeholders

I get lots of messages like this when building kea with boost-1.73.0:

/usr/include/boost/bind.hpp:36:1: note: #pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.
   36 | BOOST_PRAGMA_MESSAGE(
      | ^~~~~~~~~~~~~~~~~~~~

I suppose this warning might turn into an error with future boost releases.