This is a copy of the $ROOTSYS/montecarlo/pythia6 folder, which is not supported by ROOT from 6.32 onward.
The only difference is the CMakeList.txt, which builds with
ROOT_GENERATE_DICTIONARY
instead of
ROOT_STANDARD_LIBRARY_PACKAGE
The CMAKE build requires
-
ROOT (which is looked for automatically by find_package(ROOT))
-
pythia6 (which is provided as cmake -DPythia6_ROOT=/your/path/pythia6/)