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.
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.