Define PYTHIA8 enviroment if not defined

tutorials/CMakeLists.txt

@@ -163,6 +163,11 @@ if (NOT ROOT_pythia6_FOUND)
 endif()
 if (NOT ROOT_pythia8_FOUND)
   set(pythia_veto ${pythia_veto} pythia/pythia8.C)
+else()
+  if("$ENV{PYTHIA8}" STREQUAL "")
+    get_filename_component(pythia8dir "${PYTHIA8_INCLUDE_DIR}" DIRECTORY)
+    list(APPEND ROOT_environ PYTHIA8=${pythia8dir})
+  endif()
 endif()
 
 #---These ones are disabled !!! ------------------------------------