boost::future_errc not usable

Compiler: gcc 9.3.0, clang++ 9.0.1 / c++17 mode
Boost: 1.73

```
#include
auto future_errc_compile_error(const boost::future_error& e)
{
return e.code() == boost::future_errc::broken_promise;
}
...
```

This fails with

> /usr/include/boost/system/error_code.hpp:595:32: error: 'make_error_code' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
> /usr/include/boost/thread/futures/future_error_code.hpp:47:5: note: 'boost::system::error_code boost::system::make_error_code(boost::future_errc)' declared here, later in the translation unit
> 47 | make_error_code(future_errc e) BOOST_NOEXCEPT