COMP: only set ITK_FUTURE_LEGACY_REMOVE flag if ITK has ITK_LEGACY_REMOVE

Make sure that BRAINS build configuration matches the way ITK has
been built.

http://na-mic.org/Bug/view.php?id=3071

Note: this should really be done through a #cmakedefine in a
Configure.h.in file.
Merge pull request #26 from pieper/slicer3071-no-legacy-remove

COMP: only set ITK_FUTURE_LEGACY_REMOVE flag if ITK has ITK_LEGACY_REMOV...