This was introduced in 2.6.9-rc1

$ make V=1 O=/tmp/kbuild2/

  gcc -Wp,-MD,arch/x86_64/boot/tools/.build.d -Iarch/x86_64/boot -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer  -o arch/x86_64/boot/tools/build /build1/tduffy/openib-work/linux-2.6.9-rc1-openib/arch/x86_64/boot/tools/build.c
cc1: No such file or directory: opening dependency file arch/x86_64/boot/tools/.build.d
make[2]: *** [arch/x86_64/boot/tools/build] Error 1
make[1]: *** [bzImage] Error 2
make: *** [_all] Error 2