root: enable `clad` for automatic differentiation in ROOT
Clad is a clang plugin that can be used in Cling to create gradients of
C++ functions.
It is included in ROOT via `ExternalProject_Add`:
https://github.com/root-project/root/blob/master/interpreter/cling/tools/plugins/clad/CMakeLists.txt
Clad is used for example in RooFit to create analytical gradients of
likelihoods.