Fix system-wide install when using ROOT6
ROOT6 changed the interpreter to use LLVM. This change requires the header files to be available when loading shared libraries containing ROOT-dictionaries created by rootcint/rootcling. Also rootcint/rootcling now generates additional .pcm module files which have to be installed besides the .so file.
This commit installs the required header files in $PREFIX/share/hdtv/include and adds that path to the ROOT_INCLUDE_PATH environment variable, which controlls which directories are searched.