(inline from session; full untrimmed compiler output of the successful build)
$ cd /usr/src/sys/bus/u4b/serial/hx5424 && make
touch opt_usb.h
awk -f dragonfly/tools/makeobjops.awk -- -h dragonfly/kern/device_if.m
awk -f dragonfly/tools/makeobjops.awk -- -h dragonfly/kern/bus_if.m
cc  -O2 -pipe   -D_KERNEL -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith -Wcast-qual  -Wold-style-definition -Wmissing-include-dirs  -Wno-pointer-sign -Winit-self -Wundef -Wno-unused-but-set-variable -std=gnu11 -Werror -DKLD_MODULE -nostdinc  -I. -Idragonfly -I/usr/src/sys/bus/u4b/serial/hx5424/forwarder_x86_64/include -Idragonfly/../include -I/usr/include  -Wold-style-declaration -fno-common -ffreestanding -fno-stack-protector -fno-strict-aliasing -fno-strict-overflow -fno-asynchronous-unwind-tables -fno-omit-frame-pointer -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse -msoft-float -mno-fp-ret-in-387 -mindirect-branch=thunk-inline   -c uftdi_cfgstall_harness.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv   -r -Wl,-d -o uftdicfgstall.ko uftdi_cfgstall_harness.o
-rw-r--r-- 1 root wheel 8512 ... uftdicfgstall.ko