Xcode: Use config-specific object library files on link lines

We can do this only with Xcode 5 and above where we list the object
library files in the per-config link line value.  On older Xcode
versions we list the object files as sources so that dependencies work
correctly, but that does not allow per-config objects.  (Xcode may allow
per-config source exclusion but only by base name.)

Source/cmGlobalXCodeGenerator.cxx

@@ -2673,7 +2673,7 @@ void cmGlobalXCodeGenerator::AddDependAndLinkInformation(cmXCodeObject* target)
       std::string linkObjs;
       const char* sep = "";
       std::vector<cmSourceFile const*> objs;
-      gt->GetExternalObjects(objs, "");
+      gt->GetExternalObjects(objs, configName);
       for (std::vector<cmSourceFile const*>::const_iterator oi = objs.begin();
            oi != objs.end(); ++oi) {
         if ((*oi)->GetObjectLibrary().empty()) {