Work around a bug in clang and older versions of gcc

http://hydra.nixos.org/build/46597440

https://llvm.org/bugs/show_bug.cgi?id=28096

src/libutil/util.cc

@@ -1246,7 +1246,7 @@ std::unique_ptr<InterruptCallback> createInterruptCallback(std::function<void()>
     res->it = interruptCallbacks->end();
     res->it--;
 
-    return res;
+    return std::unique_ptr<InterruptCallback>(res.release());
 }
 
 }