Meson looks for subprojects in subprojects/ by default. Since Autotools is no longer a resident here, the submodule should be moved to the subdirectory, instead of being linked to. https://bugzilla.gnome.org/show_bug.cgi?id=780501
We will use it for in app notifications. https://bugzilla.gnome.org/show_bug.cgi?id=743630