Put 4221 back to fix MAC build warnings: Fix compiler warning on mac.  _AIR_PTR_DIFF_T_CNV should just use the normal 32/64 bit mechanism now that mac supports both.
Put 4221 back to fix MAC build warnings: Fix compiler warning on mac.  _AIR_PTR_DIFF_T_CNV should just use the normal 32/64 bit mechanism now that mac supports both.

git-svn-id: 9e9401559e51101c165cdce4d49b411eb20436ed@4416 3d70eeeb-363e-0410-a505-8a46323a89f2

BRAINSia/teem,BRAINSia/teem,BRAINSia/teem,Slicer/teem,Slicer/teem,Slicer/teem,Slicer/teem,BRAINSia/teem,BRAINSia/teem,Slicer/teem