Fix order of getting CMake variable for propagation to ITK

Add documentation to 64bits ids detection

Change-Id: Ib2e78bf6a91b3cda0f8646eed23a02764bdf6844
Merge topic 'SIMPLEITK-472_Enable64BitsIDSOnWindows' into next

57b62445 Fix order of getting CMake variable for propagation to ITK