Add new macro BOOST_NO_CXX17_HDR_OPTIONAL.

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

checks/Jamfile.v2

@@ -1,6 +1,6 @@
 #
 # *** DO NOT EDIT THIS FILE BY HAND ***
-# This file was automatically generated on Wed Apr 18 20:03:40 2018
+# This file was automatically generated on Sat Aug  4 19:11:09 2018
 #  by libs/config/tools/generate.cpp
 # Copyright John Maddock.
 # Use, modification and distribution are subject to the 
@@ -112,6 +112,7 @@ obj cxx14_return_type_deduction : test_case.cpp : <define>TEST_BOOST_NO_CXX14_RE
 obj cxx14_std_exchange : test_case.cpp : <define>TEST_BOOST_NO_CXX14_STD_EXCHANGE ;
 obj cxx14_variable_templates : test_case.cpp : <define>TEST_BOOST_NO_CXX14_VARIABLE_TEMPLATES ;
 obj cxx17_fold_expressions : test_case.cpp : <define>TEST_BOOST_NO_CXX17_FOLD_EXPRESSIONS ;
+obj cxx17_hdr_optional : test_case.cpp : <define>TEST_BOOST_NO_CXX17_HDR_OPTIONAL ;
 obj cxx17_if_constexpr : test_case.cpp : <define>TEST_BOOST_NO_CXX17_IF_CONSTEXPR ;
 obj cxx17_inline_variables : test_case.cpp : <define>TEST_BOOST_NO_CXX17_INLINE_VARIABLES ;
 obj cxx17_iterator_traits : test_case.cpp : <define>TEST_BOOST_NO_CXX17_ITERATOR_TRAITS ;

checks/test_case.cpp

@@ -1,4 +1,4 @@
-//  This file was automatically generated on Wed Apr 18 20:03:40 2018
+//  This file was automatically generated on Sat Aug  4 19:11:09 2018
 //  by libs/config/tools/generate.cpp
 //  Copyright John Maddock 2002-4.
 //  Use, modification and distribution are subject to the 
@@ -516,6 +516,11 @@
 #     error "Defect macro BOOST_NO_CXX17_FOLD_EXPRESSIONS is defined."
 #  endif
 #endif
+#ifdef TEST_BOOST_NO_CXX17_HDR_OPTIONAL
+#  ifdef BOOST_NO_CXX17_HDR_OPTIONAL
+#     error "Defect macro BOOST_NO_CXX17_HDR_OPTIONAL is defined."
+#  endif
+#endif
 #ifdef TEST_BOOST_NO_CXX17_IF_CONSTEXPR
 #  ifdef BOOST_NO_CXX17_IF_CONSTEXPR
 #     error "Defect macro BOOST_NO_CXX17_IF_CONSTEXPR is defined."