Add pybind11 support to the superbuild.
This MR is connected to https://kwgitlab.kitware.com/buildbot/buildbot/merge_requests/1048, since it will hereafter require that either shiboken or pybind11 be enabled for CMB to build.
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This MR is connected to https://kwgitlab.kitware.com/buildbot/buildbot/merge_requests/1048, since it will hereafter require that either shiboken or pybind11 be enabled for CMB to build.