netbsd 6 build trouble

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I can successfully build a netbsd-6 release on netbsd-current/amd64, but
if I try

  cd /usr/src/netbsd-6/src
  sh build.sh -j4 -O/usr/obj -R/usr/src/netbsd-6/releasedir -x release

on today's 6.1_STABLE, the build repeatedly fails at:

--- dependall-../external/lgpl3/gmp/lib/libgmp ---
--- nand_n.s ---
/usr/obj/tooldir.NetBSD-6.1_STABLE-amd64/bin/nbm4 
-DCONFIG_TOP_SRCDIR=/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp/../../dist
 -I/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp 
-I/usr/obj/external/lgpl3/gmp/lib/libgmp/dummy 
-I/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp/../../dist/ 
-I/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp/../../dist/mpn 
-DOPERATION_nand_n 
/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp/../../dist/mpn/x86_64/logops_n.asm
 > nand_n.s
nbm4: 
/usr/src/netbsd-6/src/external/lgpl3/gmp/lib/libgmp/../../dist/mpn/x86_64/logops_n.asm
 at line 20: include(../config.m4): No such file or directory

AFAICT both systems have the same netbsd-6 checkout, and equivalent
build.sh incantations.

Any guesses as to what might be different?

Cheers,

Patrick