Test that cfg_if from prelude can be overriden

Author: gnzlbg

src/libcore/tests/cfg_if_override.rs

@@ -0,0 +1,16 @@
+#![feature(cfg_if)]
+
+macro_rules! cfg_if {
+    (()) => {
+        mod foo {
+            fn foo() {}
+        }
+    }
+}
+
+cfg_if!{}
+
+#[test]
+fn it_works() {
+    foo::foo();
+}