libtask can be directly compiled from the code on Russ's site[1]. I've a copy locally compiled up, as of about 6 months ago.

For libthread, I'd suggest trying Plan 9 from User Space[2,3]. It has been a while since I played with that, but it is included as part of it.

  [1] https://swtch.com/libtask/
  [2] https://9fans.github.io/plan9port/
  [3] https://github.com/9fans/plan9port