Make it possible to pass existing CLAD_SOURCE_DIR

This would address two annoyances I have at the same time:

  1. One can now clone both `root` and `clad`, linking the source
     directories together. This makes it much easier to try out changes
     in clad in the context of ROOT.

  2. ROOT can now configure and build without an internet connection,
     even with `Dclad=ON`. This partially addresses #11603.

tools/plugins/clad/CMakeLists.txt

@@ -52,14 +52,20 @@ endif(LLVM_FORCE_USE_OLD_TOOLCHAIN)
 list(APPEND _clad_extra_cmake_args -DCLAD_BUILD_STATIC_ONLY=ON)
 
 # Wrap download, configure and build steps in a script to log output
-set(_clad_cmake_logging_settings
+set(_clad_extra_settings
   LOG_DOWNLOAD ON
   LOG_CONFIGURE ON
   LOG_BUILD ON
   LOG_INSTALL ON
+  LOG_OUTPUT_ON_FAILURE ON
 )
 
-list(APPEND _clad_cmake_logging_settings LOG_OUTPUT_ON_FAILURE ON)
+# If the CLAD_SOURCE_DIR variable is defined in the CMake configuration, we're
+# skipping the download of the repository and use the passed directory.
+if (DEFINED CLAD_SOURCE_DIR)
+  list(APPEND _clad_extra_settings DOWNLOAD_COMMAND "")
+  list(APPEND _clad_extra_settings SOURCE_DIR ${CLAD_SOURCE_DIR})
+endif()
 
 #list(APPEND _clad_patches_list "patch1.patch" "patch2.patch")
 #set(_clad_patch_command
@@ -88,7 +94,7 @@ ExternalProject_Add(
   BUILD_COMMAND ${CMAKE_COMMAND} --build . ${EXTRA_BUILD_ARGS}
   INSTALL_COMMAND ${CMAKE_COMMAND} --build . ${EXTRA_BUILD_ARGS} --target install
   BUILD_BYPRODUCTS ${CLAD_BYPRODUCTS}
-  ${_clad_cmake_logging_settings}
+  ${_clad_extra_settings}
   # We need the target clangBasic to be built before building clad. However, we
   # support building prebuilt clang and adding clangBasic breaks this case.
   # Delegate the dependency resolution to the clingInterpreter target (which