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

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

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+/*!
+@file
+Forward declares `boost::hana::keys`.
+
+@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_KEYS_HPP
+#define BOOST_HANA_FWD_KEYS_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    // Note: This function is documented per datatype/concept only.
+    //! @cond
+    template <typename T, typename = void>
+    struct keys_impl : keys_impl<T, when<true>> { };
+    //! @endcond
+
+    struct keys_t {
+        template <typename Map>
+        constexpr auto operator()(Map&& map) const;
+    };
+
+    constexpr keys_t keys{};
+
+    //! Returns a `Sequence` containing the name of the members of
+    //! the data structure.
+    //! @ingroup group-Struct
+    //!
+    //! Given a `Struct` object, `keys` returns a `Sequence` containing the
+    //! name of all the members of the `Struct`, in the same order as they
+    //! appear in the `accessors` sequence.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/struct/keys.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto keys = [](auto&& object) {
+        return implementation_defined;
+    };
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_KEYS_HPP