Properly detect debug build in slots (#3630)

`debug-assertions` can also be enabled in for release builds. This
introduces a new build-script that extracts the build type from the
`PROFILE` env variable and sets the `build_type` cfg.

substrate/core/consensus/slots/src/lib.rs

@@ -209,7 +209,7 @@ pub trait SimpleSlotWorker<B: BlockT> {
 			if slot_after_building != slot_number {
 				info!("Discarding proposal for slot {}; block production took too long", slot_number);
 				// If the node was compiled with debug, tell the user to use release optimizations.
-				#[cfg(debug_assertions)]
+				#[cfg(build_type="debug")]
 				info!("Recompile your node in `--release` mode to mitigate this problem.");
 				telemetry!(CONSENSUS_INFO; "slots.discarding_proposal_took_too_long";
 					"slot" => slot_number,
@@ -397,7 +397,7 @@ impl<T: Clone> SlotDuration<T> {
 			Some(v) => <T as codec::Decode>::decode(&mut &v[..])
 				.map(SlotDuration)
 				.map_err(|_| {
-					::client::error::Error::Backend({
+					client::error::Error::Backend({
 						error!(target: "slots", "slot duration kept in invalid format");
 						format!("slot duration kept in invalid format")
 					})