[boost] Question on DLL import/export

Reading http://www.boost.org/more/separate_compilation.html I see suggestion that all library sources start with: #define BOOST_WHATEVER_SOURCE #include <boost/whatever.hpp> Is there any particular reason why this define should be placed in source, and not in Jamfile for the library? E.g: dll boost_program_options : ...some sources... : <define>BOOST_PROGRAM_OPTIONS_SOURCE ..... ; - Volodya