Check stricter for presence of xtrans (with option to disable)

Not having xtrans available during build results in gnome-sessions
listening on remote TCP sockets (although not acting upon them).

We strongly encourage the use of xtrans to minimize this surface. If
using xtrans is not an option, it can be overruled by --without-xtrans.

https://bugzilla.gnome.org/show_bug.cgi?id=725100