Add BOOST_NO_CXX17_HDR_VARIANT.

See https://github.com/boostorg/config/issues/264

test/config_info.cpp

@@ -1161,6 +1161,7 @@ void print_boost_macros()
    PRINT_MACRO(BOOST_NO_CXX17_FOLD_EXPRESSIONS);
    PRINT_MACRO(BOOST_NO_CXX17_HDR_OPTIONAL);
    PRINT_MACRO(BOOST_NO_CXX17_HDR_STRING_VIEW);
+   PRINT_MACRO(BOOST_NO_CXX17_HDR_VARIANT);
    PRINT_MACRO(BOOST_NO_CXX17_IF_CONSTEXPR);
    PRINT_MACRO(BOOST_NO_CXX17_INLINE_VARIABLES);
    PRINT_MACRO(BOOST_NO_CXX17_ITERATOR_TRAITS);
@@ -1247,6 +1248,7 @@ void print_boost_macros()
 
 
 
+
    // END GENERATED BLOCK
 
    PRINT_MACRO(BOOST_INTEL);