If you want to build Tvheadend from source, please visit this page. Please note: that wiki development site only reflects the work in HEAD (-current). It should not deviate much should you want to build a released version from source, but you never know.
Tvheadend is part of the HTS project and is hosted at https://www.lonelycoder.com/redmine/projects/tvheadend.
It functions primarily as a TV backend for the Showtime Media player, but can be used standalone for other purposes.