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

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

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+/*!
+@file
+Forward declares `boost::hana::comparing`.
+
+@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_COMPARING_HPP
+#define BOOST_HANA_FWD_COMPARING_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns a function performing `equal` after applying a transformation
+    //! to both arguments.
+    //! @ingroup group-Comparable
+    //!
+    //! `comparing` creates an equivalence relation based on the result of
+    //! applying a function to some objects, which is especially useful in
+    //! conjunction with algorithms that accept a custom predicate that must
+    //! represent an equivalence relation.
+    //!
+    //! Specifically, `comparing` is such that
+    //! @code
+    //!     comparing(f) == equal ^on^ f
+    //! @endcode
+    //! or, equivalently,
+    //! @code
+    //!     comparing(f)(x, y) == equal(f(x), f(y))
+    //! @endcode
+    //!
+    //! @note
+    //! This is not a tag-dispatched method (hence it can't be customized),
+    //! but just a convenience function provided with the `Comparable` concept.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a Logical `Bool` and a Comparable `B`, the signature is
+    //! @f$ \mathtt{comparing} : (A \to B) \to (A \times A \to Bool) @f$.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/comparing.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto comparing = [](auto&& f) {
+        return [perfect-capture](auto&& x, auto&& y) {
+            return equal(f(forwarded(x)), f(forwarded(y)));
+        };
+    };
+#else
+    struct comparing_t {
+        template <typename F>
+        constexpr auto operator()(F&& f) const;
+    };
+
+    constexpr comparing_t comparing{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_COMPARING_HPP