Rather than building from source, there is a much quicker way of installing ntop on CentOS 5 or RHEL. You can do it via yum! Just add the following file: /etc/yum.repos.d/dag.repo:

[dag] name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
gpgkey=http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt
enabled=1

Then just run:

yum install ntop -y