As epel7 mingw-gcc has been updated since mingw-libtheora was last built, the latter can no longer be rebuilt. This can be fixed by merging at least b3df9de into epel7, build, and update.
mingw-libtheora-1.1.1-3.el7 has been submitted as an update to Fedora EPEL 7. https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2015-6d548ade73
mingw-libtheora-1.1.1-3.el7 has been pushed to the Fedora EPEL 7 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with $ su -c 'yum --enablerepo=epel-testing update mingw-libtheora' You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2015-6d548ade73
mingw-libtheora-1.1.1-3.el7 has been pushed to the Fedora EPEL 7 stable repository. If problems still persist, please make note of it in this bug report.