gnu.png and line-layout.png missing

Trying to build groff from the git repository:

$ git pull
$ ./bootstrap --no-git --gnulib-srcdir=$GNULIB_SRCDIR
$ ./configure
$ make
...
  GEN      lib/wchar.h
  GEN      lib/wctype.h
  GEN      doc/groff.html
groff.texi:12253: warning: @image file `line-layout' (for HTML) not found, using `line-layout.png'
groff.texi:26753: warning: @image file `gnu' (for HTML) not found, using `gnu.png'
cp: cannot stat './doc/gnu.png': No such file or directory
cp: cannot stat './doc/line-layout.png': No such file or directory
make: *** [Makefile:20050: doc/groff.html] Error 1

If I need to use either "make -k" or "./configure --enable-maintainer-mode",
there's something wrong with the build system. (I am not a maintainer of groff.)

Bruno