syntax: implement cfg!() which evaluates to true/false where #[cfg] would keep/remove.

Example:

   if cfg!(test) {
      calculation_to_run_only_when_testing();
   }

src/libsyntax/syntax.rs

@@ -70,6 +70,7 @@ pub mod ext {
     }
 
 
+    pub mod cfg;
     pub mod fmt;
     pub mod env;
     pub mod bytes;