This eliminates portability issues among platforms that don't have a pthreads implementation.
There was one situation in TestTlsGlobals.py where it seems the intention is to compile the executable with C. In that case I left it alone, and we will need to eventually provide a windows-based implementation of that test. But for tests which don't explicitly require pure C for the test, I don't see a good reason not to use std threads across the board.