[system] build failure when BOOST_SYSTEM_NO_DEPRECATED defined

[Richard Webb <richard.webb@…>]

The system library fails to build when BOOST_SYSTEM_NO_DEPRECATED is defined, because error_code.cpp does a

using namespace boost::system::posix_error;

and posix_error doesn't exist if BOOST_SYSTEM_NO_DEPRECATED is defined. Should perhaps be

using namespace boost::system::errc;

instead?

[Beman Dawes]

(In [53460]) System: fix #3113