System: provide optional throws function rather than throws object

[SVN r50705]

src/error_code.cpp

@@ -411,11 +411,13 @@ namespace boost
   namespace system
   {
 
+# ifndef BOOST_SYSTEM_THROWS_FUNCTION
     BOOST_SYSTEM_DECL error_code throws; // "throw on error" special error_code;
                                          //  note that it doesn't matter if this
                                          //  isn't initialized before use since
                                          //  the only use is to take its
                                          //  address for comparison purposes
+# endif
 
     BOOST_SYSTEM_DECL const error_category & get_system_category()
     {