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/cmcppdap/src/jsoncpp_json_serializer.cpp

@@ -16,7 +16,7 @@
 
 #include "null_json_serializer.h"
 
-#include <json/json.h>
+#include <cm3p/json/json.h>
 #include <cstdlib>
 #include <memory>
 

Utilities/cmcppdap/src/jsoncpp_json_serializer.h

@@ -19,7 +19,7 @@
 #include "dap/serialization.h"
 #include "dap/types.h"
 
-#include <json/forwards.h>
+#include <cm3p/json/forwards.h>
 
 namespace dap {
 namespace json {