Connect building of ports growth graph to site build process. It'll be
available at http://www.FreeBSD.org/ports/growth/status.png after next
site rebuild. Since gnuplot(1) is used, generation of this graph is
off by default and can be enabled by explicitly set -DWITH_PORTS_GROWTH
make option.

In case if graph building is enabled your copy of ports/index.html will point
to your own version of graph, otherwise link to www.FreeBSD.org will be used.