Force DEBUG_FILES to be off when building with /usr/share/mk. Otherwise
debug symbols are split out and extra files are generated that are not
expected by the plist. This is after the base system enabled DEBUG_FILES
by default in r291955.

I have pending patches to handle debug symbol splitting for ports, that will
benefit all ports, not just the ones using /usr/share/mk.

With hat: portmgr
Sponsored by: EMC / Isilon Storage Division