Replace the relative path to the po directory with a toplevel definition of po_dir and use that instead. https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/507
Meson is on track to replace autotools as the build system of choice, so support it in addition to autotools. If all goes well, we'll eventually be able to drop the latter ... https://bugzilla.gnome.org/show_bug.cgi?id=783229