ile.inc1:482: warning: The src.conf WITHOUT_CLEAN option can now be used instead of NOCLEAN."

is emitted (but the build continues and the warning seems harmless)

After stopping the build and adding WITHOUT_CLEAN to /etc/src.conf
and starting it again like so:
'make -j2 buildworld'

the build fails with

make: /etc/src.conf:7: Invalid line "WITHOUT_CLEAN"
         in /usr/src/share/mk/src.sys.mk:24
         in /usr/src/share/mk/local.sys.mk:58
         in /usr/src/share/mk/sys.mk:283
         make: Fatal errors encountered -- cannot continue

--