Add new macro BOOST_NO_CXX17_HDR_OPTIONAL.

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

include/boost/config/stdlib/libstdcpp3.hpp

@@ -299,6 +299,7 @@ extern "C" char *gets (char *__s);
 #if (BOOST_LIBSTDCXX_VERSION < 70100) || (__cplusplus <= 201402L)
 #  define BOOST_NO_CXX17_STD_INVOKE
 #  define BOOST_NO_CXX17_STD_APPLY
+#  define BOOST_NO_CXX17_HDR_OPTIONAL
 #endif
 
 #if defined(__has_include)