Why :O?

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

In the 4.0 sys/conf/Makefile.kern.inc, I find

SYSTEM_DEP?=    Makefile ${SYSTEM_OBJ:O} .gdbinit

Why is the :O there?  It causes .o files to be built in alphabetical
order, leading to hopping all over the directory tree relatively
randomly instead of tending to stick with, say, dev/pci/, until
everything (or almost) from there has been built.  This seems like a
lose to me, but presumably someone put it there for a reason.  So, my
question is, what is that reason?

This also makes it impossible to tell how far through a build is by
simply looking up the current file to the list of .o files in OBJS=,
which is how I happened to notice it, but that's a comparatively minor
issue; it's not exactly difficult to sort that list.

/~\ The ASCII                           der Mouse
\ / Ribbon Campaign
 X  Against HTML                mouse%rodents-montreal.org@localhost
/ \ Email!           7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B