"nm-autoptr.h" is done in a way that allows you to copy the header
in your source tree to support older versions of libnm, that didn't
contain the header yet. For example, we might want to use it in
network-manager-applet, but we don't want to bump the libnm dependency
to 1.11.2+ only to get this functionality.

Note that G_DEFINE_AUTOPTR_CLEANUP_FUNC() was added in glib 2.43.4,
and requires compiler support for the cleanup attribute. The compiler
support is taken as given, because we rely on it already. However,
NetworkManager and network-manager-applet still don't depend on a glib
version recent enough to provide these macros. To actually use them
(*inside*) NetworkManager/network-manager-applet, we either would have
to bump the glib minimal dependency, or reimplement g_autoptr in
/shared/nm-utils/nm-glib.h compat header.

https://bugzilla.gnome.org/show_bug.cgi?id=794294