This commit allows specifying LIBCXX_ENABLE_PARALLEL_ALGORITHMS when configuring libc++ in CMake. When that option is enabled, libc++ will assume that the PSTL can be found somewhere on the CMake module path, and it will provide the C++17 parallel algorithms based on the PSTL (that is assumed to be available).
The commit also adds support for running the PSTL tests as part of the libc++ test suite.