build: Add soup2 option

!1940 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.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1966>

js/ui/environment.js

@@ -12,6 +12,8 @@ imports.gi.versions.TelepathyGLib = '0.12';
 imports.gi.versions.TelepathyLogger = '0.2';
 
 try {
+    if (Config.HAVE_SOUP2)
+        throw new Error('Soup3 support not enabled');
     const Soup_ = imports.gi.Soup;
 } catch (e) {
     imports.gi.versions.Soup = '2.4';