Kent reported that on MAcOSX 10.7.5, despite the fact there is 10.8 SDK available, building targeting 10.8 fails.

From Kent:
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To confirm:

I'm using CMake 2.8.11.2, OS X 10.5, Xcode 4.6.3

I was having trouble compiling -- specifically configuring Slicer after
the prerequisites were built -- there was some complaint about internal
environment variables and such.

After I started over clean, I couldn't get itk to build because the little
source generator program in ITK/Modules/ThirdParty/tiff was crashing.
With CMAKE_OSX_DEPLOYMENT_TARGET=10.8, on 10.7.5, the result were
executables that couldn't run.

Setting CMAKE_OSX_DEPLOYMENT_TARGET to 10.7 (it was defaulting to 10.8)
got ITK building, but the Tk build failed trying to find X11 headers.

Setting Slicer_USE_PYTHONQT_WITH_TCL=NO got TCL to build, and now Slicer
builds to completion.
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