CMake Build Doesn't Activate XERCES_MFC_SUPPORT

The XERCES_MFC_SUPPORT define is needed for Visual Studio debug builds, it activates some code in XMemory.hpp/cpp

Should just be a case of detecting MSVC in the main CMakeLists.txt and adding a '#cmakedefine XERCES_MFC_SUPPORT' line in Xerces_autoconf_config.hpp.cmake.in