To process the hdtbl example files, eqn(1) is used, so build it first.

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -11545,7 +11545,7 @@ $(hdtblexample_DATA): $(HDTBLSTRIPFILES)
 	&& sed -e "s|[@]fontdir[@]|$(fontdir)|" \
 	       -e "s|[@]EGREP[@]|$(EGREP)|" $< >$@
 
-$(HDTBLPROCESSEDEXAMPLEFILES): gnu.eps groff troff pic tbl \
+$(HDTBLPROCESSEDEXAMPLEFILES): gnu.eps groff troff eqn pic tbl \
   contrib/hdtbl/examples/common.roff
 
 $(HDTBLSTRIPFILES): $(top_srcdir)/tmac/strip.sed