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

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

@@ -0,0 +1,49 @@
+/*!
+@file
+Forward declares `boost::hana::first`.
+
+@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_FIRST_HPP
+#define BOOST_HANA_FWD_FIRST_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the first element of a pair.
+    //! @ingroup group-Product
+    //!
+    //! Note that if the `Product` actually stores the elements it contains,
+    //! `hana::first` is required to return a lvalue reference, a lvalue
+    //! reference to const or a rvalue reference to the first element, where
+    //! the type of reference must match that of the pair passed to `first`.
+    //! If the `Product` does not store the elements it contains (i.e. it
+    //! generates them on demand), this requirement is dropped.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/first.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto first = [](auto&& product) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename P, typename = void>
+    struct first_impl : first_impl<P, when<true>> { };
+
+    struct first_t {
+        template <typename Pair>
+        constexpr decltype(auto) operator()(Pair&& pair) const;
+    };
+
+    constexpr first_t first{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_FIRST_HPP