update CMake threads behavior

[bashbaug]

See: https://cmake.org/cmake/help/latest/module/FindThreads.html

We should make a few small updates to modernize the CMake threads behavior:

1. We should use the Threads::Threads target vs. ${CMAKE_THREAD_LIBS_INIT}.
2. We should investigate whether we should set THREADS_PREFER_PTHREAD_FLAG to TRUE.