feature-flag os, but leave it enabled by default

Cargo.toml

@@ -6,3 +6,7 @@ build = "build.rs"
 
 [build-dependencies]
 gcc = "0.3.3"
+
+[features]
+default = ["os"]
+os = []

src/lib.rs

@@ -20,4 +20,6 @@ pub mod context;
 pub mod stack;
 
 mod arch;
+
+#[cfg(feature = "os")]
 pub mod os;