Now that gnome-session's acceleration helper can print the renderer
under Wayland, launch it locally. We need to launch it locally as
Wayland is not available at the time gnome-session would launch the
helper, as there's no Wayland compositor yet.
Note that this code expects the gnome-session helper scripts to live
in $libexecdir, but distributions can use
--with-gnome-session-libexecdir=DIR to pass another one.