Buildworld failure with read-only /usr/src

Browsed the archives, did not see this.  cvsup with tag=. a few
minutes ago.  Ran another cvsup just to make sure I was not in the
middle of a commit.

   /usr/src is NFS mounted r/o
   /usr/obj is NFS mounted r/w

Looks like /usr/src/include/Makefile is trying to delete a few
files, rather than using cvsup to do it, which seems wrong...

% make buildworld
   [...]
--------------------------------------------------------------
>>> stage 2.1: cleaning up the object tree
--------------------------------------------------------------
cd /usr/src; MAKEOBJDIRPREFIX=/usr/obj  MACHINE_ARCH=i386  MACHINE=i386  CPUTYPE=  GROFF_BIN_PATH=/usr/obj/usr/src/i386/legacy/usr/bin  GROFF_FONT_PATH=/usr/obj/usr/src/i386/legacy/usr/share/groff_font  GROFF_TMAC_PATH=/usr/obj/usr/src/i386/legacy/usr/share/tmac  DESTDIR=/usr/obj/usr/src/i386  _SHLIBDIRPREFIX=/usr/obj/usr/src/i386  INSTALL="sh /usr/src/tools/install.sh"  PATH=/usr/obj/usr/src/i386/legacy/usr/sbin:/usr/obj/usr/src/i386/legacy/usr/bin:/usr/obj/usr/src/i386/legacy/usr/games:/usr/obj/usr/src/i386/usr/sbin:/usr/obj/usr/src/i386/usr/bin:/usr/obj/usr/src/i386/usr/games:/sbin:/bin:/usr/sbin:/usr/bin /usr/obj/usr/src/make.i386/make -f Makefile.inc1 par-cleandir
===> share/info
===> include
rm -f osreldate.h version vers.c
rm: osreldate.h: Read-only file system
rm: version: Read-only file system
rm: vers.c: Read-only file system
*** Error code 1

Stop in /usr/src/include.