-- var='1' before my_add_subdirectory
-- var='2' in my_add_subdirectory before add_subdirectory
-- var='2' in subdirectory before set
-- var='3' in subdirectory after set
-- var_sub='' in subdirectory after set PARENT_SCOPE
-- var='2' in my_add_subdirectory after add_subdirectory
-- var_sub='sub' in my_add_subdirectory after add_subdirectory
-- var='1' after my_add_subdirectory
-- var_sub='' after my_add_subdirectory
-- var='3' taken from subdirectory