stabilize build system: depends, installer, boost/bdb fixes, cross targets groundwork

depends/x86_64-pc-linux-gnu/include/boost/hana/back.hpp

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+/*!
+@file
+Defines `boost::hana::back`.
+
+@copyright Louis Dionne 2013-2017
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_BACK_HPP
+#define BOOST_HANA_BACK_HPP
+
+#include <boost/hana/fwd/back.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/iterable.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/length.hpp>
+
+#include <cstddef>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs>
+    constexpr decltype(auto) back_t::operator()(Xs&& xs) const {
+        using It = typename hana::tag_of<Xs>::type;
+        using Back = BOOST_HANA_DISPATCH_IF(back_impl<It>,
+            hana::Iterable<It>::value
+        );
+
+    #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Iterable<It>::value,
+        "hana::back(xs) requires 'xs' to be an Iterable");
+    #endif
+
+        return Back::apply(static_cast<Xs&&>(xs));
+    }
+    //! @endcond
+
+    template <typename It, bool condition>
+    struct back_impl<It, when<condition>> : default_ {
+        template <typename Xs>
+        static constexpr decltype(auto) apply(Xs&& xs) {
+            constexpr std::size_t len = decltype(hana::length(xs))::value;
+            static_assert(len > 0, "hana::back(xs) requires 'xs' to be non-empty");
+            return hana::at_c<len - 1>(static_cast<Xs&&>(xs));
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_BACK_HPP