Extract errc into its own header

test/errc_test.cpp

@@ -0,0 +1,18 @@
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/errc.hpp>
+#include <boost/system/is_error_condition_enum.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+int main()
+{
+    BOOST_TEST_EQ( static_cast<int>( boost::system::errc::success ), 0 );
+    BOOST_TEST_EQ( static_cast<int>( boost::system::errc::no_such_file_or_directory ), ENOENT );
+    BOOST_TEST_EQ( static_cast<int>( boost::system::errc::address_family_not_supported ), EAFNOSUPPORT );
+
+    BOOST_TEST( boost::system::is_error_condition_enum< boost::system::errc::errc_t >::value );
+
+    return boost::report_errors();
+}