[pthreadpool] update sources and support find_package#17196

Merged

strega-nil-ms merged 4 commits intomicrosoft:masterfrom luncliff:port/pthreadpool

Apr 30, 2021

+46-10

Commits on Apr 10, 2021

Commits on Apr 14, 2021

[pthreadpool] prefix with 'unofficial'
luncliff
committed