Include boost/config.hpp before testinG BOOST_HAS_PRAGMA_ONCE.

include/boost/container/flat_map.hpp

@@ -10,6 +10,10 @@
 #ifndef BOOST_CONTAINER_FLAT_MAP_HPP
 #define BOOST_CONTAINER_FLAT_MAP_HPP
 
+#ifndef BOOST_CONFIG_HPP
+#  include <boost/config.hpp>
+#endif
+
 #if defined(BOOST_HAS_PRAGMA_ONCE)
 #  pragma once
 #endif