Default to builddir != srcdir

Enable a separate buildroot under XDG_CACHE_HOME.

We need to ensure that building GNOME with jhbuild exercises the same
build and code paths as building GNOME with Continuous.

We also need to ensure that modules distcheck more easily.

See discussion on desktop-devel-list:

https://mail.gnome.org/archives/desktop-devel-list/2016-February/msg00070.html

https://bugzilla.gnome.org/show_bug.cgi?id=652475