CMake: minor: modified a warning message in the detection macro (an empty reference variable is now considered valid).
CMake: minor: modified a warning message in the detection macro (an empty reference
variable is now considered valid).

FedoraScientific/salome-kernel,FedoraScientific/salome-kernel,FedoraScientific/salome-kernel,FedoraScientific/salome-kernel