build: Add soup3 option
!1940 (merged) added support for soup 3, including a fallback to soup 2.4 where the newer version isn't available.
Unfortunately it missed that libgweather has a hidden soup dependency, and now gnome-shell fails to start if a weather location has been set up and soup 3 is available.
We don't have a good way to detect that case, so hide the soup 3 support behind a build option. Distributors are expected to switch it at the same time as libgweather.