The build varies MAKEOBJDIRPREFIX at times which
can make it difficult to tack the original OBJROOT.
If SB_OBJROOT did not come from env, set and export it
the first time we set OBJROOT.
This allows SB_OBJROOT to be used in .MAKE.META.IGNORE* variables
to tweak what should be considered as making a target out-of-date.
Update auto.obj.mk to latest from contrib/bmake - handles relative
objdirs more simply and reliably.
Also update jobs.mk and stage-install.mk