Ideally, we would chain the m4 processing, .in substitutions, and file
installation so that the commands don't have to be repeated. Unfortunately
this does not seem currently possible, because custom_target() output cannot
be fed into install_data(), so it's necessary to use the 'install',
'install_dir' arguments to control installation. Nevertheless, rework the
rules to repeat less stuff and unify handling of conditions between the
different file types.