Breaking the "bison.help: bison" dependency is non-trivial with MAKE_JOBS>1;
instead make the problematic target a NOOP.

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -9579,6 +9579,7 @@ doc/$(am__dirstamp):
 	@: > doc/$(am__dirstamp)
 
 $(srcdir)/doc/bison.info: doc/bison.texi $(srcdir)/doc/version.texi $(doc_bison_TEXINFOS)
+neutered-bison-info:
 	$(AM_V_MAKEINFO)restore=: && backupdir="$(am__leading_dot)am$$$$" && \
 	am__cwd=`pwd` && $(am__cd) $(srcdir) && \
 	rm -rf $$backupdir && mkdir $$backupdir && \