In this case use the configure option: --with-http-proxy=PROXY, where PROXY is the address of your proxy, for example: --with-http-proxy=wwwcache.university.edu:3128.
Note however that it may not be necessary: Pleinpot is using wget as a client for the http download, therefore, if wget is already installed on your machine and if you are using it, it is certainly already configured for using your proxy (there is a configuration file named .wgetrc). In that case, the option --with-http-proxy= is un-necessary.