It looks like build_environment_setup.md got renamed to
getting_started_build_tools.md in this commit:
commit e6c638bed1
Author: skullY <skullydazed@gmail.com>
Date: Sat Aug 5 20:54:34 2017 -0700
Overhaul the Getting Started section and add a FAQ section
docs/{build_environment_setup.md => getting_started_build_tools.md} | 132 ++++++++++++++++++++++++++++++++++++-------------------------------------
This commit adjusts the links to match the new name.
Roy Wellington Ⅳ
skullY
Jack Humbert