Since you focus on OSX, the solution to your problem is to create Lumberjack as a basis (as opposed to referencing the source code in your A and B modules), and then using this structure wherever it is needed (i.e. in any project using modules A or B).
Indeed, Lumberjack already includes a project that will build Lumberjack.framework, check it out: CocoaLumberjack/Xcode/LumberjackFramework/Desktop/Lumberjack.xcodeproj.
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