In commit 8a46b25c, NetworkManager
bumped its glib dependency to 2.40 or newer.

"nm-glib.h" header is precisely the compatiblity implementation that we
use to cope with older glib versions. Note, that the same implementation
is also used by applet and VPN plugins.

However, applet and VPN plugins don't yet require 2.40 glib. Also,
they don't yet require NetworkManager 1.12.0 API (which was the one
that bumped the glib requirement to 2.40). Hence, when "nm-glib.h"
is used in applet or VPN, it must still cope with older versions,
although, the code is not used by NetworkManager itself.

Partly revert 8a46b25c so that nm-glib.h
again works for 2.32 or newer.

The same is true, also for "nm-test-utils.h", which is also used by
applet and VPN plugins.