Update boost to 1.68.0

Keeps the boost libraries up to date. This also silences informational
messages that get spammed throughout the build, such as:

"Info: Boost.Config is older than your compiler version - probably
nothing bad will happen - but you may wish to look for an update Boost
version.  Define BOOST_CONFIG_SUPPRESS_OUTDATED_MESSAGE to suppress this
message."

Which makes the compilation process a lot less noisy on Windows. It's
now much easier to actually spot warnings that occur.

boost/iterator/distance.hpp

@@ -0,0 +1,65 @@
+// Copyright (C) 2017 Michel Morin.
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_ITERATOR_DISTANCE_HPP
+#define BOOST_ITERATOR_DISTANCE_HPP
+
+#include <boost/config.hpp>
+#include <boost/iterator/iterator_categories.hpp>
+#include <boost/iterator/iterator_traits.hpp>
+
+namespace boost {
+namespace iterators {
+
+    namespace detail {
+        template <typename SinglePassIterator>
+        inline BOOST_CXX14_CONSTEXPR typename iterator_difference<SinglePassIterator>::type
+        distance_impl(
+            SinglePassIterator first
+          , SinglePassIterator last
+          , single_pass_traversal_tag
+        )
+        {
+            typename iterator_difference<SinglePassIterator>::type n = 0;
+            while (first != last) {
+                ++first;
+                ++n;
+            }
+            return n;
+        }
+
+        template <typename RandomAccessIterator>
+        inline BOOST_CXX14_CONSTEXPR typename iterator_difference<RandomAccessIterator>::type
+        distance_impl(
+            RandomAccessIterator first
+          , RandomAccessIterator last
+          , random_access_traversal_tag
+        )
+        {
+            return last - first;
+        }
+    }
+
+    namespace distance_adl_barrier {
+        template <typename SinglePassIterator>
+        inline BOOST_CXX14_CONSTEXPR typename iterator_difference<SinglePassIterator>::type
+        distance(SinglePassIterator first, SinglePassIterator last)
+        {
+            return detail::distance_impl(
+                first, last, typename iterator_traversal<SinglePassIterator>::type()
+            );
+        }
+    }
+
+    using namespace distance_adl_barrier;
+
+} // namespace iterators
+
+using iterators::distance;
+
+} // namespace boost
+
+#endif