Build project when other project changes

Hi,

I've two projects:
1. Mobile app
2. Library which is used by the app

The problem is that library is not included as submodule and linked by CocoaPods. So, app project knows nothing about library integration.
But it's still important to build the app when library changes. Is that possible? Maybe, I can specify build trigger for changes in completely not related git repo (haven't find out how to do this.)