[libvirt] Fixing libvirt's libxl driver breakage -- where to define LIBXL_API_VERSION?

Hi libvirt maintainers, Xen's control library libxenlight (libxl) requires application (libvirt in this case) to explictily define LIBXL_API_VERSION. This is lacking at the moment so libvirt's libxl driver always gets the latest APIs. We changed one of the APIs in libxl so libvirt's libxl driver can't build at the moment. I want to submit a patch for libvirt to define LIBXL_API_VERSION, but I'm not sure where I should add that to. Can you give me some pointers? Or even better -- a specific file that I should edit? src/Makefile.am and src/Makfile.in are good candicate to me as far as I can tell. Thanks Wei.