Make clippy an optional dependency

sync/Cargo.toml

@@ -10,8 +10,12 @@ authors = ["Ethcore <admin@ethcore.io"]
 [dependencies]
 ethcore-util = { path = "../util" }
 ethcore = { path = ".." }
-clippy = "0.0.41"
+clippy = { version = "0.0.41", optional = true }
 log = "0.3"
 env_logger = "0.3"
 time = "0.1.34"
 rand = "0.3.13"
+
+[features]
+default = []
+dev = ["clippy", "ethcore/dev", "ethcore-util/dev"]

sync/src/lib.rs

@@ -15,9 +15,9 @@
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
 #![warn(missing_docs)]
-#![feature(plugin)]
+#![cfg_attr(feature="dev", feature(plugin))]
+#![cfg_attr(feature="dev", plugin(clippy))]
 #![feature(augmented_assignments)]
-#![plugin(clippy)]
 // Keeps consistency (all lines with `.clone()`) and helpful when changing ref to non-ref.
 #![allow(clone_on_copy)]