Don't install NPM if already installed

When I install npm from source it doesn't actually place the binary inside the nodejs install path. It creates a symlink for /lib/node_modules/npm/bin/npm-cli.js to /usr/bin/npm

I installed 1.3.6 from source on Ubuntu 12.04.2 LTS (GNU/Linux 3.2.0-23-generic-pae i686)