Make IRTKInclude.cmake work with CMake 2.6 again.
Make IRTKInclude.cmake work with CMake 2.6 again.

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