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

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

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+/*!
+@file
+Forward declares `boost::hana::length`.
+
+@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_LENGTH_HPP
+#define BOOST_HANA_FWD_LENGTH_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Return the number of elements in a foldable structure.
+    //! @ingroup group-Foldable
+    //!
+    //! Given a `Foldable` `xs`, `length(xs)` must return an object of an
+    //! unsigned integral type, or an `IntegralConstant` holding such an
+    //! object, which represents the number of elements in the structure.
+    //!
+    //! @note
+    //! Since only compile-time `Foldable`s are supported in the library
+    //! right now, `length` must always return an `IntegralConstant`.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/length.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto length = [](auto const& xs) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename = void>
+    struct length_impl : length_impl<T, when<true>> { };
+
+    struct length_t {
+        template <typename Xs>
+        constexpr auto operator()(Xs const& xs) const;
+    };
+
+    constexpr length_t length{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_LENGTH_HPP