Deprecated boost/iterator.hpp, emit warnings on inclusion.

The header defines boost::iterator template, which is an alias for
std::iterator, which is itself deprecated since C++17.

Updated test to avoid testing the definition in C++17 onwards
to avoid failures due to deprecation warnings from libc++-13.

doc/changes.qbk

@@ -10,6 +10,7 @@
 
 * Added `boost::allocator_traits`, an implementation of `std::allocator_traits`.
 * Made `boost::pointer_traits` SFINAE friendly.
+* `boost/iterator.hpp` is deprecated and will be removed in a future release. The header defines `boost::iterator` template, which is equivalent to `std::iterator` in `<iterator>` header. However, since `std::iterator` is itself deprecated in C++17, users are advised to remove `boost::iterator` or `std::iterator` use from their code.
 
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