Per-language pthread link flag

Some compilers offer a -pthread flag to use at link time instead of linking to the pthread library. The need to spell the flag differently depending on the compiler driver used has come up a few times:

cmake-developers list Sept 2019: Possible feature request concerning conditional linking support by generator expressions
CUDA support: #18008 (comment 412820)

I'm opening this issue to discuss possible approaches.

Edited Sep 26, 2019 by Brad King

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