[boost] nullptr vs 0

Compilers apparently are warning on the use of 0 as a null pointer constant, suggesting we use nullptr instead. But compilers don't know that we support C++03 where nullptr isn't a thing. Case in point: https://github.com/boostorg/throw_exception/pull/22 ifdef-ing every use of nullptr is unwieldy, so maybe we need BOOST_NULLPTR added to Boost.Config? That would expand to nullptr when it's supported, and 0 otherwise.