Software under GPL or AGPL license cannot be distributed legally via the Apple App Store:

https://www.fsf.org/news/2010-05-app-store-compliance

To allow porting Cagou to iOS, the following amendment should appended to its license:

Additional Permissions For Submission to Apple App Store: Provided that you are otherwise in compliance with the GPLv3 for each covered work you convey (including without limitation making the Corresponding Source available in compliance with Section 6 of the GPLv3), Open Whisper Systems also grants you the additional permission to convey through the Apple App Store non-source executable versions of the Program as incorporated into each applicable covered work as Executable Versions only under the Mozilla Public License version 2.0 (https://www.mozilla.org/en-US/MPL/2.0/).

from: https://github.com/signalapp/libsignal-protocol-c/commit/defc71dfaafb8eeab05db62a4bdc88d37e2ee56b