I got an odd error when using the system makefiles, and doing a make depend;

   ...output follows...
   files=" cache.C main.C ";  if [ "$files" != "  " ]; then  mkdep -a  -+  $files;
    fi
   gcc: unrecognized option `-+'
   ...end output...

Oops.  This is historical baggage left over from when mkdep(1) used to
call cpp(1) directly.  I've removed the `-+' option from bsd.dep.mk.