autogen.sh: Add a hack to work in the gnome-ostree build system

gnome-ostree doesn't support passing parameters to autogen.sh, but
we can't do the full autotools thing because we don't have yelp.

A simple hackaround for this is to check NOCONFIGURE; if someone's
specifying that we just give them the autotools always.

https://bugzilla.gnome.org/show_bug.cgi?id=697875