iPhone sdk compiler warning Directory help needed -L not found

I have a nuisance problem. I have upgraded sdk's and along the way, I must have previously used the 3.1.3 simulator, which I no longer have. I have not found where and how to delete the reference, so I can get rid of the warning

Directory '/Developer/Platforms/iPhoneSimulator.platform/Deve开发者_StackOverflow社区loper/SDKs/iPhoneSimulator3.1.3.sdk/usr/lib' following -L not found

Thanks

In Project menu Edit Project Settings under base sdk change value to 4.2