Fix rule_in_var.mk

Now we parse rules at eval time, not in parse time.
diff --git a/test/err_no_target_commands2.mk b/test/err_no_target_commands2.mk
new file mode 100644
index 0000000..47570b1
--- /dev/null
+++ b/test/err_no_target_commands2.mk
@@ -0,0 +1,2 @@
+$(empty)
+	all: