ilmbase: add mirror

Added an explicit mirror address (http://download-mirror.savannah.gnu.org) since the original url (http://download.savannah.gnu.org) redirects to an unavailable mirror site for some regions as notes below.

https://github.com/Homebrew/homebrew/commit/d4fa35b8b52a60435d634270eea6f244a4fdbfef#diff-612f93a14932da6b052b8e3cc3da8dd9