Cannot compile Slicer on Mac - macOS Sierra + clang 9 + cmake 3.9.1

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@blowekamp I confirm I have that entry:

$ grep SITK_CHECK_CXX11 CMakeCache.txt
SITK_CHECK_CXX11:INTERNAL=FALSE

But @jcfr confirmed recently c++11 is not currently supported in Slicer: Transition to VTK 8.0