We use libgdbus_example_objectmanager_dep as dependency for various
tests, but this implies only a link dependency while it doesn't ensure
that the sources are generated.
Make this explicit
When using glib as a meson subproject on Windows the build currently fails
due to too long paths during the build process. See
https://github.com/mesonbuild/meson/issues/4226 for the upstream bug.
To work around the issue shorten the filenames of the generated gdbus files.
Properly define GLIB/GOBJECT_STATIC_COMPILATION when static build is enabled.
Use library() instead of shared_library() to allow selecting static builds.
https://bugzilla.gnome.org/show_bug.cgi?id=784995