
In libnm, we prefer opaque typedefs. gtk-doc needs to be patched to properly
generate documentation. Add a check for that.
Add a test. By default, this does not fail but just prints a warning. The test
can be made failing by setting NMTST_CHECK_GTK_DOC=1.
See-also: https://gitlab.gnome.org/GNOME/gtk-doc/merge_requests/2
(cherry picked from commit 02464c052e
)