[cmake] prevent usage of some shell commands if not present
[cmake] prevent usage of some shell commands if not present

siconos/siconos,siconos/siconos,radarsat1/siconos,radarsat1/siconos,siconos/siconos,siconos/siconos,radarsat1/siconos,radarsat1/siconos,radarsat1/siconos