How should I test error codes with

I have this code (very similar to what is suggested here that throws an exception:

int accept4(int sockfd, sockaddr *addr, socklen_t *addrlen, int flags)
{
   const int fd = ::accept4(sockfd, addr, addrlen, flags);
   if (fd < 0)
   {
       const auto tmp = errno;
       throw ::std::system_error(tmp, ::std::system_category(), "accept4(2)");
   }
   else
   {
      return fd;
   }
}

And this code for testing for a particular exception reason:

catch (const ::std::system_error &e)
{
   static const auto block_err = ::std::system_error(EWOULDBLOCK,
                                                     ::std::system_category());

   const auto ecode = e.code(); 

   // EWOULBLOCK is fine, everything else, re-throw it.
   if (block_err.code() != ecode)
   {
      throw;
   }
}

This seems kind of needlessly verbose and not quite the right way to do things. There's this whole idea of generic errors and a whole enum (see ::std::errc) full of them along with some kind of system that's supposed to convert between system specific error codes and these generic errors.

Is there a better, more approved way of handling this?