One cannot build the gnumeric documentation without a live X display.  At
some point during the documentation build process, make enters subdirectory
"doc/C" and attempts to build "func.defs" using the following command:

    LC_ALL="C" ; export LC_ALL ; ../../src/gnumeric \
    --dump-func-defs="func.defs"

If $DISPLAY is not set or is invalid, an error along the following lines
appears:

    (gnumeric:17865): Gtk-WARNING **: cannot open display:

It would be nice if gnumeric could avoid trying to set up its X connection
for non-interactive operations such as this.  That would facilitate batch
rebuilds in headless environments, such as on large compute servers or out
of batch jobs.