Change the specification for native ordering to allow implementations to support orderings in addition to big and little. Thanks to Howard Hinnant for suggesting this change and providing draft wording. Also update headers to reflect actual inclusion hierarchy.

include/boost/endian/conversion.hpp

@@ -23,7 +23,6 @@ namespace boost
 {
 namespace endian
 {
-#ifndef BOOST_ENDIAN_ORDER_ENUM_DEFINED
   BOOST_SCOPED_ENUM_START(order)
   {
     big, little,
@@ -33,8 +32,6 @@ namespace endian
       native = little
 # endif
   }; BOOST_SCOPED_ENUM_END
-# define BOOST_ENDIAN_ORDER_ENUM_DEFINED
-#endif
 
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