Peculiar build failure

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

I'm trying to build 4.0.1 and I'm getting peculiar build failures.  The
weird thing is, I've done this plenty often enough before without any
problem.  I _am_ crossbuilding from 3.1, but that shouldn't cause any
trouble, surely?

They appear to be due to attempts to write into the source tree, which
fails because the user doing the build does not have permission to
write into the source.  Specifically, the build ticks along nicely
until getting to tools/gmake.  Then:

dependall ===> tools/gmake
...lots of configure messages...
...build messages for glob/...
rm -f libglob.a
ar cru libglob.a glob.o fnmatch.o 
ranlib libglob.a
Making all in config
Making all in po
Making all in doc
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-1: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-2: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-3: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-4: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-5: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-6: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-7: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-8: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-9: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-10: Permission 
denied
rm: /usr/src.401/tools/gmake/../../gnu/dist/gmake/doc/make.info-11: Permission 
denied

*** Failed target:  make.info

and this failure cascades up the chain of makes.

The status lines at the beginning of the log are

===> build.sh command: ./build.sh -D /home/mouse/i386-4.0.1/DESTDIR -O 
/home/mouse/i386-4.0.1/OBJDIR -U build
===> build.sh started: Mon Dec 21 01:01:27 EST 2009
===> NetBSD version:   4.0.1
===> MACHINE:          i386
===> MACHINE_ARCH:     i386
===> Build platform:   NetBSD 3.1 i386
===> HOST_SH:          /bin/sh

I can provide more information, even to the point of the whole build
log, if it would help.

I can't believe this is how it's supposed to work (and indeed it hasn't
in various other builds of 4.0.1 I've done), so I must have done
something wrong somewhere.  Any idea what?

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