cppdap: Build as part of CMake or use external installation

Add `cm3p/` headers to use the selected copy of the library.

Co-authored-by: Glen Chung <kuchung@microsoft.com>

Utilities/cm3p/cppdap/optional.h

@@ -0,0 +1,11 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+/* Use the cppdap library configured for CMake.  */
+#include "cmThirdParty.h"
+#ifdef CMAKE_USE_SYSTEM_CPPDAP
+#  include <dap/optional.h> // IWYU pragma: export
+#else
+#  include <cmcppdap/include/dap/optional.h> // IWYU pragma: export
+#endif