-current 'groff' build failure on -7 and -8 hosts?

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

I've been seeing failures of recent -current building 'groff' on my
netbsd-7 and -8 amd64 hosts:

[...]
--- lex.o ---
c++ -I. -I/x/current/src/external/gpl2/groff/dist/src/preproc/eqn  -I/r0/build/current/obj/amd64/tools/groff/build/src/include -I/x/current/src/tools/groff/../../external/gpl2/groff/dist/src/include -DHAVE_CONFIG_H -O -I/r0/build/current/tools/amd64/include/compat -I/x/current/src/tools/compat  -DHAVE_NBTOOL_CONFIG_H=1 -D_FILE_OFFSET_BITS=64 -c  /x/current/src/external/gpl2/groff/dist/src/preproc/eqn/lex.cpp
/x/current/src/external/gpl2/groff/dist/src/preproc/eqn/lex.cpp:25:21: fatal error: eqn_tab.h: No such file or directory
 #include "eqn_tab.h"
                     ^
compilation terminated.
[...]

The netbsd-7 host is building to empty (removed) obj/tool/DEST/REL
directories.  The netbsd-8 host is doing an update build from which
"obj/<arch>/external/bsd/acpica" "obj/<arch>/external/gpl2/groff", and
"obj/<arch>/tools/groff" were removed due to previous observed failures
building these.

The problem with 'groff' originally only appeared in a distribution/release
build, so I nuked the "obj/<arch>/tools" version as well and now it doesn't
build either.

In a separate tree synced with HEAD, I build only tools and GENERIC.
Since the tools were already built, it wasn't updated, so this build
succeeds.

I recall some stuff with 'eqn' not long ago, but only recently tried
rebuilding -current again.  I don't see what's going wrong, especially
when building to empty (removed) output directories.

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