fully refactor runtime-std to use conditional compilation

substrate/runtime-std/build.rs

@@ -0,0 +1,14 @@
+//! Set a nightly feature
+
+extern crate rustc_version;
+use rustc_version::{version, version_meta, Channel};
+
+fn main() {
+    // Assert we haven't travelled back in time
+    assert!(version().unwrap().major >= 1);
+
+    // Set cfg flags depending on release channel
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        println!("cargo:rustc-cfg=feature=\"nightly\"");
+    }
+}