enable multi-threading

[h-yon]

Just a suggestion.

[CLAassistant]

All committers have signed the CLA.

[tkralphs]

Interesting idea. I guess it would need to be made a bit more bullet-proof, though. Just Googling briefly, it seems this is not guaranteed to return something non-zero and also may not be supported by older compilers. See, e.g., https://stackoverflow.com/questions/27971195/handle-stdthreadhardware-concurrency.