better yacc rule in Makefile

if y.tab.c is older than y.tab.h, make
interprets doing nothing as a failure,
because y.tab.c hasn't been updated.
so update it.

R=r
DELTA=4  (0 added, 0 deleted, 4 changed)
OCL=15615
CL=15622

src/cmd/6a/Makefile

@@ -33,7 +33,7 @@ y.tab.h: $(YFILES)
 	yacc $(YFLAGS) $(YFILES)
 
 y.tab.c: y.tab.h
-	# do nothing
+	test -f y.tab.c && touch y.tab.c
 
 clean:
 	rm -f $(OFILES) $(TARG) *.6 enam.c 6.out a.out y.tab.h y.tab.c

src/cmd/cc/Makefile

@@ -42,7 +42,7 @@ y.tab.h: $(YFILES)
 	yacc $(YFLAGS) $(YFILES)
 
 y.tab.c: y.tab.h
-	# do nothing
+	test -f y.tab.c && touch y.tab.c
 
 clean:
 	rm -f $(OFILES) *.6 enam.c 6.out a.out y.tab.h y.tab.c $(LIB)

src/cmd/gc/Makefile

@@ -37,7 +37,7 @@ y.tab.h: $(YFILES)
 	yacc $(YFLAGS) $(YFILES)
 
 y.tab.c: y.tab.h
-	# do nothing
+	test -f y.tab.c && touch y.tab.c
 
 sysimport.c:	sys.go mksys.bash
 	bash mksys.bash