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

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

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+/*!
+@file
+Forward declares `boost::hana::power`.
+
+@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_POWER_HPP
+#define BOOST_HANA_FWD_POWER_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Elevate a ring element to its `n`th power.
+    //! @ingroup group-Ring
+    //!
+    //! Specifically, `power(x, n)`, is equivalent to multiplying `x` with
+    //! itself `n` times using the Ring's multiplication. If the power is
+    //! equal to `zero`, the Ring's identity (`one`) is returned.
+    //!
+    //! @param x
+    //! A `Ring` element that is elevated to its `n`th power.
+    //!
+    //! @param n
+    //! A non-negative `IntegralConstant` representing the power to which `x`
+    //! is elevated.
+    //!
+    //!
+    //! @note
+    //! Only the tag of `x` is used for tag-dispatching.
+    //!
+    //! Example
+    //! -------
+    //! @include example/power.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto power = [](auto&& x, auto const& n) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename R, typename = void>
+    struct power_impl : power_impl<R, when<true>> { };
+
+    struct power_t {
+        template <typename X, typename N>
+        constexpr decltype(auto) operator()(X&& x, N const& n) const;
+    };
+
+    constexpr power_t power{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_POWER_HPP