message: Make use of libarchive optional
  The CMake option was already present, just not used.
diff: @@ -113,7 +113,6 @@ add_subdirectory(networkdatabases)
  add_subdirectory(openbabel)
  add_subdirectory(quantumoutput)
  add_subdirectory(playertool)
  -add_subdirectory(plugindownloader)
  add_subdirectory(povray)
  add_subdirectory(select)
  add_subdirectory(selectiontool)
  @@ -129,6 +128,10 @@ if(USE_MOLEQUEUE)
  add_subdirectory(scriptfileformats)
  endif()
  +if(USE_LIBARCHIVE)
  + add_subdirectory(plugindownloader)
  +endif()
  +
  # The scene plugins
  add_subdirectory(ballandstick)
  add_subdirectory(licorice)
  