Use CMake WARNING message to display warning regarding nvcc nullptr missing.
Use CMake WARNING message to display warning regarding nvcc nullptr missing.

BioMedIA/irtk-legacy,sk1712/IRTK,ghisvail/irtk-legacy,sk1712/IRTK,BioMedIA/IRTK,BioMedIA/irtk-legacy,ghisvail/irtk-legacy,BioMedIA/IRTK,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,sk1712/IRTK,sk1712/IRTK,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,BioMedIA/IRTK,BioMedIA/IRTK