Below is what I get when the build reaches that source file.
(the attached file is the relevant `.config` file)
gcc -D__KERNEL__ -I/home/karrde/linux-2.5.5-pre1/include -Wall
-Wstrict-prototypes -Wno-trigraphs -O2 -fomit-frame-pointer
-fno-strict-aliasing -fno-common -pipe -mpreferred-stack-boundary=2
-march=i686 -DKBUILD_BASENAME=setup -c -o setup.o setup.c
setup.c: In function `setup_arch':
setup.c:682: `rd_image_start' undeclared (first use in this function)
setup.c:682: (Each undeclared identifier is reported only once
setup.c:682: for each function it appears in.)
setup.c:683: `rd_prompt' undeclared (first use in this function)
setup.c:684: `rd_doload' undeclared (first use in this function)
make[1]: *** [setup.o] Error 1
make[1]: Leaving directory
`/home/karrde/linux-2.5.5-pre1/arch/i386/kernel'
make: *** [_dir_arch/i386/kernel] Error 2