Only build cockpit-ws when requirements are met

We need Glib 2.37.4 in order to do reliable TLS, which
is a requirement of cockpit-ws

https://bugzilla.gnome.org/show_bug.cgi?id=701283
https://bugzilla.redhat.com/show_bug.cgi?id=1143943

We also need libssh 0.6.0 to build cockpit-ws

Disable cockpit-ws on platforms that cannot support it.
We can connect to such servers via the 'Add Server' functionality