install_prereq: For PJProject, point users to configure script.

The installation script and the new configure option --with-pjproject-bundled
aimed to accomplish the same. However, the installation script was out of
date. Users should go for the maintained configure option, or the Wiki.

ASTERISK-24598

Change-Id: Icbf4b562f81f7c05bd24a3805bd46c0beb4ebd44