ENH: Allow to configure with CMake 2.8.10

In previous revision, IntensitySegmenter could only be configured with CMake 2.8.12 and above. This was because "ExternalData" and "ExternalData_Add_Test" only exist starting in CMake 2.8.11 and CMake 2.8.12 is already widely used. However, CMake 2.8.10 is still used to compile Slicer extensions for Windows and Linux.

http://www.nitrc.org/plugins/scmsvn/viewcvs.php?view=rev&root=dentaltools&revision=11
Merge pull request #754 from fbudin69500/update-IntensitySegmenter

ENH: Allow to configure with CMake 2.8.10