How to make a Mac app made in 10.6 work in 10.5

I recently made an app on a Mac running OS X 10.6. I now want that app to work on 10.5.x systems. I changed 开发者_StackOverflow中文版the target build in xcode from 10.6 to 10.5 in the project settings which led to some code changes. But when I try it on 10.5 it just shows a "stop-sign" on the app-icon:

These are my Xcode build settings:

What should I do?

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Did you cross-compile your app 32-bit (and ppc if you are supporting PPC 10.5 machines)?