cmake: test CMP0046 before setting
cmake: test CMP0046 before setting

Fixes CentOS 7 build

vcrhonek/openwsman,kkaempf/openwsman,Openwsman/openwsman,Openwsman/openwsman,vcrhonek/openwsman,Openwsman/openwsman,Openwsman/openwsman,kolbma/openwsman,kkaempf/openwsman,Openwsman/openwsman,Openwsman/openwsman,kolbma/openwsman,kkaempf/openwsman,Openwsman/openwsman,kkaempf/openwsman,kolbma/openwsman,kkaempf/openwsman,vcrhonek/openwsman,kolbma/openwsman,kolbma/openwsman,vcrhonek/openwsman,kkaempf/openwsman,kolbma/openwsman,vcrhonek/openwsman,vcrhonek/openwsman,kkaempf/openwsman,vcrhonek/openwsman,kolbma/openwsman