YARP_manager library installation does no longer require CMake 2.8.10
Set YARP_EXPERIMENTAL_MANAGER_LIBRARY=TRUE in experimental builds
Rename libraries created in external folder using prefix "YARP_priv_"

Update YarpDescribe to filter out YARP_priv libraries from the
YARP_LIBRARIES variable.

Also install tinyxml, if YARP_EXPERIMENTAL_MANAGER_LIB is enabled.
This is required because YARP_manager requires that target that is not
in the export list.