Erroring out on parser conflicts

While hacking on parser/gram.y just now I noticed in passing that the
automatically generated ecpg parser had 402 shift/reduce conflicts.
(Don't panic, the parser in CVS is fine.) If you don't pay very close
attention, it is easy to miss this. Considering also that we frequently
have to educate contributors that parser conflicts are not acceptable,
should we try to error out if we see conflicts?

Something like this could work:

$(srcdir)/preproc.c: $(srcdir)/preproc.y
$(BISON) -d $(BISONFLAGS) -o $@ $< 2>preproc.stderr
cat preproc.stderr
[ ! -s preproc.stderr ]