This is most likely what is being referenced: https://github.com/jart/cosmopolitan/blob/master/Makefile

I like how the includes are separated and commented.

Also if you weren't already familiar with their work you might be interested in giving this a read: https://justine.lol/ape.html

Can

  % grep '^include' Makefile -c 
  159

includes with

  % wc --lines --total=only $(awk '/^include/{ print $2 }' Makefile) Makefile 
  22547

lines in total really count as having a single makefile? I have no dog in this fight, just wondering.