get this makefile to execute the mkdep commands - no need for a submake
and xargs at that point.
However we do need to do something to stop the 'mkdep -d' and CLEANDEPEND
command lines being to long.
Note that 'echo ${xxx} | cmd' is ok because echo is a shell builtin, and
isn't (usually) subject to the kernel's command line limits.
04e3bf3418