ntop is failing to configure after r414458 updated rrdtool to version
1.6.0. ntop depends on librrd_th.so (threading enabled shlib) which
has been removed from the latest rrdtool package -- threading is now
always enabled.

Quick fix: make ntop depend on the older rrdtool12 port.

Longer term fix: investigate 'ntopng'

http://www.ntop.org/products/traffic-analysis/ntop/

PR: 209230