CMake: add rapidxml to project
CMake: add rapidxml to project

For better integration in Visual Studio generated projects.

otris/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,bkircher/ews-cpp