Build failed on fresh sources

[Greywolf <relayer%gmail.com@localhost>]

Sources from 2019-09-26, building a kernel, I get this:

###
gcc -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse -mno-avx
-msoft-float -mindirect-branch=thunk-inline -mindirect-branch-register
-ffreestanding -fno-zero-initialized-in-bss
-fno-delete-null-pointer-checks -g -O2 -fno-omit-frame-pointer
-fstack-protector -Wstack-protector --param ssp-buffer-size=1
-fno-strict-aliasing -fno-common -std=gnu99 -Werror -Wall -Wno-main
-Wno-format-zero-length -Wpointer-arith -Wmissing-prototypes
-Wstrict-prototypes -Wold-style-definition -Wswitch -Wshadow
-Wcast-qual -Wwrite-strings -Wno-unreachable-code -Wno-pointer-sign
-Wno-attributes -Wextra -Wno-unused-parameter -Wold-style-definition
-Wno-sign-compare -Wno-shadow -Damd64 -Dx86_64 -I.
-I../../../../external/bsd/acpica/dist
-I../../../../external/bsd/libnv/dist
-I../../../../../common/lib/libx86emu
-I../../../../../common/lib/libc/misc -I../../../../../common/include
-I../../../../arch -I../../../.. -nostdinc -DCOMPAT_UTILS -DDIAGNOSTIC
-DCOMPAT_44 -D_KERNEL -D_KERNEL_OPT -std=gnu99
-I../../../../lib/libkern/../../../common/lib/libc/quad
-I../../../../lib/libkern/../../../common/lib/libc/string
-I../../../../lib/libkern/../../../common/lib/libc/arch/x86_64/string
-D_FORTIFY_SOURCE=2 -I../../../../external/bsd/common/include
-I../../../../external/bsd/common/include
-I../../../../external/bsd/drm2/include
-I../../../../external/bsd/drm2/include
-I../../../../external/bsd/common/include
-I../../../../external/bsd/drm2/dist/include
-I../../../../external/bsd/drm2/dist/include/drm
-I../../../../external/bsd/drm2/dist/uapi
-I../../../../external/bsd/drm2/dist -D__KERNEL__
-DCONFIG_BACKLIGHT_CLASS_DEVICE=0
-DCONFIG_BACKLIGHT_CLASS_DEVICE_MODULE=0
-DCONFIG_DRM_FBDEV_EMULATION=1 -DCONFIG_FB=0
-I../../../../../common/include -I../../../../external/bsd/libnv/dist
-I../../../../external/bsd/drm2/i915drm
-I../../../../external/bsd/drm2/dist/drm/i915
-DCONFIG_DRM_I915_FBDEV=1 -DCONFIG_DRM_I915_PRELIMINARY_HW_SUPPORT=0
-DCONFIG_DRM_FBDEV_EMULATION=1
-I../../../../external/bsd/acpica/dist/include -c
../../../../external/bsd/drm2/dist/drm/i915/intel_ddi.c -o intel_ddi.o
In file included from
../../../../external/bsd/drm2/dist/drm/i915/intel_bios.h:33:0,
                 from ../../../../external/bsd/drm2/dist/drm/i915/i915_drv.h:50,
                 from
../../../../external/bsd/drm2/dist/drm/i915/intel_ddi.c:35:
../../../../external/bsd/drm2/dist/drm/i915/intel_ddi.c: In function
'translate_signal_level':
../../../../external/bsd/drm2/dist/include/drm/drmP.h:241:6: error:
this statement may fall through [-Werror=implicit-fallthrough=]
   if (unlikely(drm_debug & DRM_UT_KMS))   \
      ^
../../../../external/bsd/drm2/dist/drm/i915/intel_ddi.c:2229:3: note:
in expansion of macro 'DRM_DEBUG_KMS'
   DRM_DEBUG_KMS("Unsupported voltage swing/pre-emphasis level: 0x%x\n",
   ^~~~~~~~~~~~~
../../../../external/bsd/drm2/dist/drm/i915/intel_ddi.c:2231:2: note: here
  case DP_TRAIN_VOLTAGE_SWING_LEVEL_0 | DP_TRAIN_PRE_EMPH_LEVEL_0:
  ^~~~
cc1: all warnings being treated as errors
*** Error code 1

Stop.
###

It's one of a bug, a coding error, or user error.  I will accept all
of these answers, but if it's the third one, I'd like to know where
I've gone wrong.
-- 
--*greywolf;