DF-5636 rig build (guest, root) -- final successful build:
$ cd /root/df5636 && sh ./build.sh
  ... (forwarding headers + makeobjops for device_if/bus_if/iicbb_if) ...
  cc -O2 -pipe -D_KERNEL -Wall ... -Werror -DKLD_MODULE ... -c build_rig/df5636_rig.c
  cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o df5636_rig.ko df5636_rig.o
  cc -O2 -pipe -D_KERNEL -Wall ... -Werror -DKLD_MODULE ... -c build_reg/iicbb_reg.c
  cc -Wl,--build-id=sha1 -nostdlib ... -r -Wl,-d -o iicbb_reg.ko iicbb_reg.o
  cc -O2 -Wall -o df5636ctl df5636ctl.c
  /usr/obj/root/df5636/build_reg/iicbb_reg.ko
  /usr/obj/root/df5636/build_rig/df5636_rig.ko
  df5636ctl
All objects compiled warning-free under -Werror.

Fix build (guest, root):
$ patch -p1 < fix.diff -C /usr/src/sys/bus/smbus/smbconf.c   (3 hunks OK)
$ cd /usr/src/sys/bus/smbus/smbbus -> (module path not usable: smbus is compiled
  into X86_64_GENERIC, config line 335)
$ cd /usr/src && make -j4 quickkernel KERNCONF=X86_64_GENERIC   (smbconf.o rebuilt)
$ make installkernel KERNCONF=X86_64_GENERIC && reboot          (kernel #3)