Good Question. On Linux it seems, by default they are not built at all.
Are they still relevant? Does somebody use them?
I always use configure to build cb on linux. Neither their names, nor EXTRA nor DIST are mentioned in configure --help.
devpak_plugin is the only one, where the directory (devpak_plugin) name does not correspond to the plugin name (devpakupdater). That naming is inconsistent. In addition to that, context matters. It already resides below the directory named plugins.