message: RooUnfold: add `WITH_ROOUNFOLD` definition
  When a library links to the `RooUnfold::RooUnfold` CMake target, the compiler is
  invoked with `-DWITH_ROOUNFOLD`, allowing source code to have conditional
  sections:
  #ifdef WITH_ROOUNFOLD
  ...
  #endif
  Note: it has to be verified whether `rootcling` is automatically getting those
  definitions as well.
diff: @@ -35,7 +35,8 @@ find_package_handle_standard_args(RooUnfold DEFAULT_MSG
  add_library(RooUnfold::RooUnfold SHARED IMPORTED)
  set_target_properties(RooUnfold::RooUnfold PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${ROOUNFOLD_INCLUDE_DIR}"
  - IMPORTED_LOCATION "${ROOUNFOLD_LIBPATH}")
  + IMPORTED_LOCATION "${ROOUNFOLD_LIBPATH}"
  + INTERFACE_COMPILE_DEFINITIONS "WITH_ROOUNFOLD")
  # Unset RooUnfold variables
  unset(ROOUNFOLD_LIBPATH)
  