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

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

@@ -0,0 +1,48 @@
+/*!
+@file
+Forward declares `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_FWD_BACK_HPP
+#define BOOST_HANA_FWD_BACK_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the last element of a non-empty and finite iterable.
+    //! @ingroup group-Iterable
+    //!
+    //! Given a non-empty and finite iterable `xs` with a linearization of
+    //! `[x1, ..., xN]`, `back(xs)` is equal to `xN`. Equivalently, `back(xs)`
+    //! must be equivalent to `at_c<N-1>(xs)`, and that regardless of the
+    //! value category of `xs` (`back` must respect the reference semantics
+    //! of `at`).
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/back.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto back = [](auto&& xs) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename It, typename = void>
+    struct back_impl : back_impl<It, when<true>> { };
+
+    struct back_t {
+        template <typename Xs>
+        constexpr decltype(auto) operator()(Xs&& xs) const;
+    };
+
+    constexpr back_t back{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_BACK_HPP