iphone: get User Defined variable in Target’s setting by code?

As the Info.plist file is preprocessed too, you can use this approach:

Define a User defined setting in your build settings, for Example CLASS_NAME.
And a key to your Info.plist-file. Name the key CLASS_NAME and set the value to ${CLASS_NAME}.

You can then access this setting by:

NSString* className = [[[NSBundle mainBundle] infoDictionary] valueForKey:@"CLASS_NAME"];