pthreadpool is a pthread-based thread pool implementation. It is intended to
provide functionality of #pragma omp parallel for for POSIX systems where OpenMP
is not available.

Features:
* C interface (C++-compatible)
* Run on user-specified or auto-detected number of threads
* Work-stealing scheduling for efficient work balancing
* Compatible with Linux, macOS, and Native Client environments
* Covered with unit tests and microbenchmarks