$ kldload drm
$ cd /tmp/df4918 && make
Warning: Object directory not changed from original /tmp/df4918
dragonfly -> /usr/src/sys
machine_base -> /usr/src/sys/platform/pc64
machine -> /usr/src/sys/platform/pc64/include
cpu_base -> /usr/src/sys/cpu/x86_64
cpu -> /usr/src/sys/cpu/x86_64/include
cc  -O2 -pipe -I/usr/src/sys/dev/drm/include -I/usr/src/sys/dev/drm/include/drm -I/usr/src/sys/dev/drm/include/uapi -include /usr/src/sys/dev/drm/kconfig.h -DKBUILD_MODNAME=\"df4918\"  -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 ... -c df4918.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv   -r -Wl,-d -o df4918.ko df4918.o
== full untrimmed output of the successful build (no warnings beyond the objdir notice above; -Werror clean)