When BOOST_SP_REPORT_IMPLEMENTATION is defined, report what platform-specific atomic implementation is used

test/sp_report_implementation.cpp

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+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#define BOOST_SP_REPORT_IMPLEMENTATION
+#include <boost/shared_ptr.hpp>
+#include <boost/smart_ptr/detail/spinlock.hpp>
+#include <boost/smart_ptr/detail/atomic_count.hpp>
+
+int main()
+{
+}