Skip implicit rule for .rbinc when BASERUBY is unavailable

Author: nobu

common.mk

@@ -1318,7 +1318,7 @@ preludes: {$(VPATH)}miniprelude.c
 	$(ECHO) making $@
 	$(Q) $(MINIRUBY) $(tooldir)/mk_rbbin.rb $(SRC_FILE) > $(OS_DEST_FILE)
 
-{$(srcdir)}.rb.rbinc:
+{$(srcdir)}.rb.$(HAVE_BASERUBY:yes=)rbinc:
 	$(ECHO) making $@
 	$(Q) $(BASERUBY) $(tooldir)/mk_builtin_loader.rb $(DUMP_AST) $(SRC_FILE)