The C++ driver errors out when you try to build it for Windows at text.h, while trying to compile text.cpp. It hits a #error for toNativeString() when _UNICODE is undefined, presumably because we haven't decided yet how we want to get from UTF-8 to the native code page?
To repro, build the client source distribution archive:
Unpack the archive, which is named mongo-cxx-driver.zip.
Then, run scons inside the unpacked driver source directory: