Add new macro BOOST_NO_CXX17_HDR_OPTIONAL.

Fixes: https://github.com/boostorg/config/issues/236.

test/config_info.cpp

@@ -1159,6 +1159,7 @@ void print_boost_macros()
    PRINT_MACRO(BOOST_NO_CXX14_STD_EXCHANGE);
    PRINT_MACRO(BOOST_NO_CXX14_VARIABLE_TEMPLATES);
    PRINT_MACRO(BOOST_NO_CXX17_FOLD_EXPRESSIONS);
+   PRINT_MACRO(BOOST_NO_CXX17_HDR_OPTIONAL);
    PRINT_MACRO(BOOST_NO_CXX17_IF_CONSTEXPR);
    PRINT_MACRO(BOOST_NO_CXX17_INLINE_VARIABLES);
    PRINT_MACRO(BOOST_NO_CXX17_ITERATOR_TRAITS);
@@ -1237,6 +1238,7 @@ void print_boost_macros()
 
 
 
+
    // END GENERATED BLOCK
 
    PRINT_MACRO(BOOST_INTEL);