Improve suggestion for missing fmt str in println

Avoid using `concat!(fmt, "\n")` to improve the diagnostics being
emitted when the first `println!()` argument isn't a formatting string
literal.

src/libstd/macros.rs

@@ -155,8 +155,14 @@ macro_rules! print {
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! println {
     () => (print!("\n"));
-    ($fmt:expr) => (print!(concat!($fmt, "\n")));
-    ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
+    ($fmt:expr) => ({
+        print!($fmt);
+        print!("\n");
+    });
+    ($fmt:expr, $($arg:tt)*) => ({
+        print!($fmt, $($arg)*);
+        print!("\n");
+    });
 }
 
 /// Macro for printing to the standard error.