This change is motivated by:

* the user poll https://discourse.slicer.org/t/poll-which-version-of-macos-do-you-use-to-run-slicer/6055

* the deployment target explicitly required by BRAINSTools. Note that this
  could be relaxed because it is internally only needed to find newer version
  of TBB when BRAINSABC module is enabled.
  From TBB:
  > Note that in TBB 2018 and higher we do not support OS X 10.9, and it
  > is not tested anymore. TBB team recommends you to either switch to
  > 10.11 or stay at an older version of TBB.
  > See https://github.com/conda-forge/tbb-feedstock/issues/42#issuecomment-427921828
  > and https://github.com/01org/tbb/blob/tbb_2018/doc/Release_Notes.txt

* Deployment target >= 10.11 is the one officially supported by Qt 5.10
  and Qt 5.11.
  See https://doc.qt.io/qt-5.11/supported-platforms-and-configurations.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@28019 3bd1e089-480b-0410-8dfb-8563597acbee