Using std::iterator as base class is deprecated in C++17, causes warnings in MSVC++

[vbaderks]

The following code requires an explicit macro to suppress a warning to be defined when compiling with MSVC++:

statechart/include/boost/statechart/state_machine.hpp

Line 359 in 586445b

class state_iterator : public std::iterator<

Adding _SILENCE_CXX17_ITERATOR_BASE_CLASS_DEPRECATION_WARNING solves the problem as a workaround. Other Boost libraries have fixed this issue in the past, for example: boostorg/utility#34

Can provide PR if needed.

Background info:
https://www.fluentcpp.com/2018/05/08/std-iterator-deprecated/