Commit a76b28bd moved the dbus-interfaces to the
dbus-interfaces directory without changing the
path for docs build as well, resulting in a fail
of the build when building with gtk_doc=true.
So change the path in the build accordingly to
ensure that the build does not fail when building
with gtk_doc=true.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/236
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
