Explicitly rely on pthreads if possible

Explicitly rely on pthreads if possible

allows us to set the needed stacksize on thread creation.

Useful for environments with too small a default stack size (e.g. Alpine Linux with musl).

Passed STC, no regression:

LLR: 2.96 (-2.94,2.94) {-1.25,0.25}
Total: 17816 W: 1344 L: 1275 D: 15197
Ptnml(0-2): 30, 1057, 6682, 1092, 47
https://tests.stockfishchess.org/tests/view/5f402b5587a5c3c63d8f534d

closes https://github.com/official-stockfish/Stockfish/pull/3047

fixes https://github.com/official-stockfish/Stockfish/issues/3041

No functional change.