Fix linking in objc_id

Author: madsmtm

objc_id/src/id.rs

@@ -206,6 +206,25 @@ mod tests {
     use objc::runtime::Object;
     use objc::{class, msg_send};
 
+    #[cfg(not(target_vendor = "apple"))]
+    use objc::runtime::Class;
+
+    #[cfg(not(target_vendor = "apple"))]
+    #[link(name = "gnustep-base", kind = "dylib")]
+    extern "C" {}
+
+    // TODO: Remove the need for this hack
+    #[cfg(not(target_vendor = "apple"))]
+    extern "C" {
+        static _OBJC_CLASS_NSObject: Class;
+    }
+
+    #[cfg(not(target_vendor = "apple"))]
+    #[test]
+    fn get_class_to_force_linkage() {
+        unsafe { &_OBJC_CLASS_NSObject };
+    }
+
     fn retain_count(obj: &Object) -> usize {
         unsafe { msg_send![obj, retainCount] }
     }

objc_id/src/lib.rs

@@ -11,7 +11,7 @@ which can be cloned to allow multiple references.
 
 Weak references may be created using the [`WeakId`](struct.WeakId.html) struct.
 
-```
+```no_run
 # use objc::msg_send;
 use objc::runtime::{Class, Object};
 use objc_id::{Id, WeakId};