jsoncpp: Add cm3p/ headers for json/json.h and json/forwards.h

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

Utilities/cm3p/json/forwards.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 jsoncpp library configured for CMake.  */
+#include "cmThirdParty.h"
+#ifdef CMAKE_USE_SYSTEM_JSONCPP
+#  include <json/forwards.h> // IWYU pragma: export
+#else
+#  include <cmjsoncpp/include/json/forwards.h> // IWYU pragma: export
+#endif

Utilities/cm3p/json/json.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 jsoncpp library configured for CMake.  */
+#include "cmThirdParty.h"
+#ifdef CMAKE_USE_SYSTEM_JSONCPP
+#  include <json/json.h> // IWYU pragma: export
+#else
+#  include <cmjsoncpp/include/json/json.h> // IWYU pragma: export
+#endif