use old boost::optional implementation for newer boost

fixes #777; see also 70c19e3

CMakeLists.txt

@@ -166,7 +166,7 @@ if(${Boost_VERSION} LESS "105600")
     )
 endif()
 
-if(${Boost_VERSION} EQUAL "106100")
+if(${Boost_VERSION} EQUAL "106100" OR ${Boost_VERSION} GREATER "106100")
     # with boost 1.61 some boost::optional internals were changed. However
     # boost::spirit relies on some API the old implementation provided.  This
     # define enables the usage of the old boost::optional implementation.