Use BOOST_HAS_PRAGMA_ONCE.

Remembering to first include config, so that it'll actually be defined.

[SVN r86726]

include/boost/unordered/detail/table.hpp

@@ -7,6 +7,11 @@
 #ifndef BOOST_UNORDERED_DETAIL_ALL_HPP_INCLUDED
 #define BOOST_UNORDERED_DETAIL_ALL_HPP_INCLUDED
 
+#include <boost/config.hpp>
+#if defined(BOOST_HAS_PRAGMA_ONCE)
+#pragma once
+#endif
+
 #include <boost/unordered/detail/buckets.hpp>
 #include <boost/unordered/detail/util.hpp>
 #include <boost/type_traits/aligned_storage.hpp>