CDRIVER-2083 Fix Windows/Unix CMake conditionals

Fixes two cases that were conditional on MSVC and shouldn't have been:

* Uses WIN32 for requiring ws2_32 or not
* Uses UNIX for configuring Threads or not

The latter fix just moves `find_package(Threads)` next to commands
in a later "if UNIX" stanza that uses CMAKE_THREADS_LIBS_INIT.