ns3 script cannot install to privileged directories

The block on use of sudo or root shell that has been implemented on the ns3 script prevents ./ns3 install from being able to install on a privileged directory such as /usr/local.

To reproduce:

./ns3 configure ...
./ns3 build
./ns3 install  # won't work if /usr/local is root-only
sudo ./ns3 install # won't work either

Either (or both) ./ns3 install --enable-sudo or sudo ./ns3 install should be permitted, IMO.