For normal builds, MKOBJDIRS default to `no' (not `yes').

It's only build.sh that sets this to `yes' by default.
Noted by OGAWA Takaya in [toolchain/19823]

BUILDING

@@ -294,7 +294,7 @@ CONFIGURATION
                       ject directories will be created automatically (via a
                       ``make obj'' pass) at the start of a build.
 
-                      Default: ``yes''
+                      Default: ``no''
 
      NBUILDJOBS       Now obsolete.  Use the make(1) option -j, instead (see
                       below)
@@ -563,7 +563,9 @@ BUILDING
                ting of /usr/obj will place build-time files under
                /usr/obj/bin, /usr/obj/lib, and so forth.
 
-     -o        Set the value of MKOBJDIRS to ``no''.
+     -o        Set the value of MKOBJDIRS to ``no''.  Otherwise, it will be
+               automatically set to ``yes'' (which is opposite to the default
+               behaviour).
 
      -R rel    Set the value of RELEASEDIR to rel.
 
@@ -642,4 +644,4 @@ HISTORY
 BUGS
      A few platforms are not yet using this build system.
 
-NetBSD                           May 17, 2003                               10
+NetBSD                           May 18, 2003                               10

doc/BUILDING.mdoc

@@ -1,4 +1,4 @@
-.\"	$NetBSD: BUILDING.mdoc,v 1.20 2003/05/17 08:10:39 lukem Exp $
+.\"	$NetBSD: BUILDING.mdoc,v 1.21 2003/05/18 11:18:02 lukem Exp $
 .\"
 .\" Copyright (c) 2001-2003 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -40,7 +40,7 @@
 .\" Toolchain prefix for commands
 .ds toolprefix nb
 .
-.Dd May 17, 2003
+.Dd May 18, 2003
 .Dt BUILDING 8
 .Os NetBSD
 .
@@ -544,7 +544,7 @@ Indicates whether object directories will be created automatically
 (via a
 .Dq make obj
 pass) at the start of a build.
-.DFLTy
+.DFLTn
 .
 .It Sy NBUILDJOBS
 Now obsolete.
@@ -1089,6 +1089,9 @@ Set the value of
 .Sy MKOBJDIRS
 to
 .Dq no .
+Otherwise, it will be automatically set to
+.Dq yes
+(which is opposite to the default behaviour).
 .
 .It Fl R Ar rel
 Set the value of