thinkpad-4.3-r1 doesn't build inside a chroot with a different kernel version to the running kernel. The main Makefile and the 2.4/Makefile use different variables for the module version so the emake command in src_compile needs changing to: emake DIR_MOD_VER=/lib/modules/${KV} DIR_MODULES_VER=/lib/modules/${KV} || die "Make failed" 2.4 works for me with this change. I've looked at 2.2 and 2.5 and as far as I can see without testing, this should fine for them as well.
Added your fix. Didn't bump revision, as people who already has this installed won't notice the difference and shouldn't be forced to upgrade.