Only impl std traits if feature is specified

.travis.yml

@@ -197,3 +197,4 @@ notifications:
 branches:
   only:
     - master
+    - 0.2

src/lib.rs

@@ -160,20 +160,20 @@ pub use crate::error::Error;
 #[allow(dead_code)]
 mod util;
 
-cfg_if! {
-    // Unlike the other Unix, Fuchsia and iOS don't use the libc to make any calls.
-    if #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "emscripten",
-                 target_os = "freebsd", target_os = "haiku",     target_os = "illumos",
-                 target_os = "linux",   target_os = "macos",     target_os = "netbsd",
-                 target_os = "openbsd", target_os = "redox",     target_os = "solaris"))] {
-        #[allow(dead_code)]
-        mod util_libc;
-        // Keep std-only trait definitions for backwards compatiblity
-        mod error_impls;
-    } else if #[cfg(feature = "std")] {
-        mod error_impls;
-    }
-}
+#[cfg(all(
+    unix,
+    not(any(
+        target_os = "ios",
+        target_os = "fuchsia",
+        target_os = "hermit",
+        target_os = "l4re"
+    ))
+))]
+#[allow(dead_code)]
+mod util_libc;
+
+#[cfg(feature = "std")]
+mod error_impls;
 
 // These targets read from a file as a fallback method.
 #[cfg(any(