Don't ignore pgen error (on "make Parser/pgen.stamp")
diff --git a/Makefile.pre.in b/Makefile.pre.in
index aa6d3a8..3a179f6 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -577,7 +577,7 @@
 $(GRAMMAR_H) $(GRAMMAR_C): Parser/pgen.stamp
 Parser/pgen.stamp: $(PGEN) $(GRAMMAR_INPUT)
 		-@$(INSTALL) -d Include
-		-$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C)
+		$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C)
 		-touch Parser/pgen.stamp
 
 $(PGEN):	$(PGENOBJS)