LLVM libc++ support

• Can build the entire software stack with libc++ from the LLVM project instead of libstdc++
• Add xwidget support at Jupyter Lab
• Add Jupyter kernels for C++ and CUDA (C++ 11, 14, 17) based on the kernel of the Cling project
  • https://github.com/root-project/cling/tree/master/tools/Jupyter