Previously `struct iovec` was explicitly defined on non-Unix systems,
which caused name conflicts when other libraries (e.g. Cyrus SASL) did the same.
Using `struct iovec` as `msgpack::iovec` solves the problem.
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.
Daniil Kovalev