stable: put macro debug flags behind feature gate

This enables tests to work on stable. To enable macro debugging, build
with `--features macro-debug`.

Signed-off-by: Sean Cross <sean@xobs.io>

Cargo.toml

@@ -6,6 +6,10 @@ edition = "2018"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+macro-debug = []
+default = []
+
 [dependencies]
 
 [workspace]

src/lib.rs

@@ -1,5 +1,6 @@
-#![feature(trace_macros)]
-#![feature(log_syntax)]
+#![cfg_attr(feature="macro-debug",feature(trace_macros))]
+#![cfg_attr(feature="macro-debug",feature(log_syntax))]
+#[cfg(feature="macro-debug")]
 trace_macros!{true}
 
 /*