Unbreak 'make -j' by declaring the dependency that exists between the
two generated vapi files (forcing building of the second one to wait
until after the first one has been built).
https://bugzilla.gnome.org/show_bug.cgi?id=680374
This patch adds the autotools facilities to generate vapi files so that
libnm-util and libnm-glib can be consumed from Vala.
It depends on vapigen and it is a soft dependency.
