add subst2.mk
fix subst and patsubst accespt more than 3 arguments.
fix panic error message of subst and patsubst.
diff --git a/test/subst2.mk b/test/subst2.mk
new file mode 100644
index 0000000..1686426
--- /dev/null
+++ b/test/subst2.mk
@@ -0,0 +1,11 @@
+# http://www.gnu.org/software/make/manual/make.html#Syntax-of-Functions
+,:= ,
+empty:=
+space:= $(empty) $(empty)
+foo:= a b c
+bar:= $(subst $(space),$,,$(foo))
+# bar is now `,abc'
+# space in `,$(foo)' replaced with `$', which will be empty
+
+test:
+ echo $(bar)