Step cancellation: Don't use pthread_cancel()

This was a bad idea because pthread_cancel() is unsalvageable broken in C++. Destructors are not allowed to throw exceptions (especially in C++11), but pthread_cancel() can cause a __cxxabiv1::__forced_unwind exception inside any destructor that invokes a cancellation point. (This exception can be caught but must be rethrown.) So let's just kill the builder process instead.

Created by  Eelco Dolstra  on November 7, 2016
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