Add new macro BOOST_NO_CXX17_HDR_OPTIONAL.

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

include/boost/config/compiler/visualc.hpp

@@ -202,6 +202,7 @@
 #if (_MSC_VER < 1911) || (_MSVC_LANG < 201703)
 #  define BOOST_NO_CXX17_STRUCTURED_BINDINGS
 #  define BOOST_NO_CXX17_IF_CONSTEXPR
+#  define BOOST_NO_CXX17_HDR_OPTIONAL
 #endif
 
 // MSVC including version 14 has not yet completely

include/boost/config/detail/suffix.hpp

@@ -1023,6 +1023,11 @@ namespace std{ using ::type_info; }
 #  define BOOST_NO_CXX11_FIXED_LENGTH_VARIADIC_TEMPLATE_EXPANSION_PACKS
 #endif
 
+// This is a catch all case for obsolete compilers / std libs:
+#if !defined(__has_include) || !__has_include(<optional>)
+#  define BOOST_NO_CXX17_HDR_OPTIONAL
+#endif
+
 //
 // Finish off with checks for macros that are depricated / no longer supported,
 // if any of these are set then it's very likely that much of Boost will no

include/boost/config/stdlib/libcpp.hpp

@@ -89,6 +89,7 @@
 // C++17 features
 #if (_LIBCPP_VERSION < 4000) || (__cplusplus <= 201402L)
 #  define BOOST_NO_CXX17_STD_APPLY
+#  define BOOST_NO_CXX17_HDR_OPTIONAL
 #endif
 #if (_LIBCPP_VERSION > 4000) && (__cplusplus > 201402L) && !defined(_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR)
 #  define BOOST_NO_AUTO_PTR

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)