After you have build and installed the world, you want probably to clean up your system. If you do it on the way described in the Makefile, and start make delete-old you will be prompted for configuration for each file have to be deleted.
To do a batch delete without confirmations simple set BATCH_DELETE_OLD_FILES:
make -DBATCH_DELETE_OLD_FILES delete-old
make -DBATCH_DELETE_OLD_FILES delete-old-libs
See also: