Try with: cmake -Dpythia8=ON -DPYTHIA8_DIR=<workdir>/pythia8309 /path/to/ROOT/source
If you still have problems, try to uninstall the “old” system default packages.
Try with: cmake -Dpythia8=ON -DPYTHIA8_DIR=<workdir>/pythia8309 /path/to/ROOT/source
If you still have problems, try to uninstall the “old” system default packages.