snc: fixed rules for generating parser and lexer

With parallel make (-j) the snl.c and snl.h targets were built
twice which could cause building to fail. With the changed rules
I ran into a make bug and had to add an empty recipy where it
should not be needed. Also needed to work around a bug in the base
rules (no explicit rule exists that generates the .d files).

src/snc/Makefile

@@ -46,15 +46,21 @@ include $(TOP)/configure/RULES
 vpath %.lem ..
 vpath %.lt ..
 
-snl.c snl.h: snl.lem snl.lt $(LEMON)
-	$(RM) $@
+snl.c: snl.lem snl.lt $(LEMON)
+	$(RM) snl.c snl.h
 	$(LEMON) o=. $<
 
+snl.h: snl.c
+	# empty recipy needed to work around make bug #35455
+
 vpath %.re ..
 
-lexer.c: snl.re snl.h
+lexer.c: snl.re
 	$(RE2C) -s -b -o $@ $<
 
+# work around bug in base rules
+lexer.o: snl.h
+
 $(COMMON_DIR)/snl.bnf: snl.lem
 	$(RM) $@
 	$(LEMON) o=. -g $< > $@.tmp