To remove boost dependency, `-DMSGPACK_USE_BOOST=OFF` for cmake.
By default, `-DMSGPACK_USE_BOOST` is `ON`.
NOTE: In order to build tests `-DMSGPACK_USE_BOOST=ON` is required.
For C++ compiler, the option `-DMSGPACK_NO_BOOST` is required to remove
boost.
MSVC CLI defined their own nullptr and provides for __nullptr for standard C++11.
https://msdn.microsoft.com/en-us/library/4ex65770.aspx
msgpack-c introduce MSGPACK_NULLPTR for internal use, it is defined as __nullptr only if compiled on C++ CLI otherwise defined as nullptr.
