Ubuntu 17.04 launch results in segfault if the build was done
by GCC 6.2 (works fine with GCC 4.9). Backtrace shows that it
crashes in gtk_init_check() call somewhere in libmirclient and
tests show that it works fine with GDK_BACKEND=x11.

So we use gdk_set_allowed_backends() method to explicitly state
that we support only "x11" GDK backend, that way it doesn't try
to use libmirclient and it does not crash.

Fix #3176 #3162