Applied patch in https://sft.its.cern.ch/jira/browse/ROOT-6294  Not really tested because we have no test associated to this plugin.

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>

Force a 'configuration' dependency to LLVM and CLING by deleting the build stamp file. After a change in LLVM/CLING a cmake command will trigger a re-build of them. This will trigger eventually the rebuilding of all dictionaries if rootcling_tmp has been redone. The libraries are taken from the build directory instead of the installation.