groff and mkdep?

I've just started to try and sync up to -CURRENT, and my first time
asking on the lists, and everything seems to be going fine, except
groff's build dies with
rm -f .depend
mkdep -f .depend -a       
/usr/src/gnu/usr.bin/groff/src/libs/libdriver/../../../../../../contrib/groff/src/libs/libdriver/input.cpp /usr/src/gnu/usr.bin/groff/src/libs/libdriver/../../../../../../contrib/groff/src/libs/libdriver/printer.cpp
/usr/src/contrib/groff/src/libs/libdriver/input.cpp:244:20: driver.h: No
such file or directory
/usr/src/contrib/groff/src/libs/libdriver/input.cpp:245:20: device.h: No
such file or directory
/usr/src/contrib/groff/src/libs/libdriver/printer.cpp:29:20: driver.h:
No such file or directory
mkdep: compile failed
*** Error code 1

I've tried everything I can think of, including changing the Makefile to
explicitly include  the groff directories via CFLAGS, CXXFLAGS, and such
(and it adds the directive), but that doesn't seem to help in the
slightest.
Does anyone have any idea what's wrong with this? Is there any way to
bypass this particular program in the build, and is it safe to do so?