cleanup some feature based build params

crates/async-std-resolver/src/lib.rs

@@ -123,6 +123,7 @@ pub async fn resolver(
 ///
 /// This will use `/etc/resolv.conf` on Unix OSes and the registry on Windows.
 #[cfg(any(unix, target_os = "windows"))]
+#[cfg(feature = "system-config")]
 pub async fn resolver_from_system_conf() -> Result<AsyncStdResolver, ResolveError> {
     AsyncStdResolver::from_system_conf(AsyncStdRuntimeHandle).await
 }

crates/async-std-resolver/src/tests.rs

@@ -102,6 +102,7 @@ fn test_system_lookup() {
 
 #[test]
 #[ignore]
+#[cfg(feature = "system-config")]
 // these appear to not work on CI, test on macos with `10.1.0.104  a.com`
 #[cfg(unix)]
 fn test_hosts_lookup() {