BOOST_OS_UNIX not working

#include <boost/predef.h>
#include <iostream>

int main()
{
  std::cout << "BOOST_OS_UNIX: "    << BOOST_OS_UNIX    << "\n"
            << "BOOST_OS_LINUX: "   << BOOST_OS_LINUX   << "\n"
            << "unix: "             << unix             << "\n"
            << "__unix: "           << __unix           << "\n"
            << "_XOPEN_SOURCE: "    << _XOPEN_SOURCE    << "\n"
            << "_POSIX_SOURCE: "    << _POSIX_SOURCE    << "\n";
}