Rather than generating enum classes by hand (and complaining in each
file that "this should really be standard"), use glib-mkenums.
Unfortunately, we need a very new version of glib-mkenums in order to
deal with NM's naming conventions and to fix a few other bugs, so just
import that into the source tree temporarily.
Also, to simplify the use of glib-mkenums, import Makefile.glib from
https://bugzilla.gnome.org/654395.
To avoid having to run glib-mkenums for every subdirectory of src/,
add a new "generated" directory, and put the generated enums files
there.
Finally, use Makefile.glib for marshallers too, and generate separate
ones for libnm-glib and NetworkManager.
