in the CMakeLists.txt file for libbson, ws2_32 is linked it the build is NOT UNIX, but it probably should be it is WIN32, as it is in the cdriver proper.
https://github.com/mongodb/mongo-c-driver/blob/master/src/libbson/CMakeLists.txt#L235
https://github.com/mongodb/mongo-c-driver/blob/master/CMakeLists.txt#L671