turn off YARP_manager target if CREATE_YMANAGER is off, fixes #256

Currently the YARP_manager library tries to build even if CREATE_YMANAGER is turned off, leading to #256 if tinyxml is not installed. This fix turns off all the manager things if this flag is off. Not sure if this is best solution - maybe the various flags involved need to be rationalized? Or the YARP_manager library could be unconditionally built, with a fix for the tinyxml issue to make sure the internal tinyxml version is used if needed? /cc @drdanz @pattacini