Ensure linkage of the gnustep-base

The linker is smart enough to figure out that we are not using
any symbols from the Foundation library, and elides the request to
link it. To circumvent this, we define a (unused) function referencing
the symbol for NSObject

objc_foundation/src/lib.rs

@@ -11,9 +11,23 @@ pub use self::object::{INSObject, NSObject};
 pub use self::string::{INSCopying, INSMutableCopying, INSString, NSString};
 pub use self::value::{INSValue, NSValue};
 
+#[cfg(any(target_os = "macos", target_os = "ios"))]
 #[link(name = "Foundation", kind = "framework")]
 extern "C" {}
 
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+use objc::runtime::Class;
+
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+#[link(name = "gnustep-base", kind = "dylib")]
+extern { static _OBJC_CLASS_NSObject : Class; }
+
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+#[allow(dead_code)]
+unsafe fn get_class_to_force_linkage() -> &'static Class {
+    &_OBJC_CLASS_NSObject
+}
+
 #[macro_use]
 mod macros;