Since 7f2a6f17fc ("xdg-desktop-portal-gtk: Enable all default
portals unconditionally"), building gcr_4 is necessary to build
xdg-desktop-portal-gtk, but it wasn't possible for systems that can't
use systemd.
I've called the option "systemdSupport" for consistency with gcr (3)'s
equivalent option.
744ab7a50e