csound FTBFS with -Werror=format-security because it sets -Wno-format and the combination of those two is considered an error by gcc. Adding -Wformat to customC*FLAGS doesn't fix it, then it fails with errors raised by -Werror=format-security, there seems to be quite a few of them in the source.
csound-6.03.2-1.fc21 has been submitted as an update for Fedora 21. https://admin.fedoraproject.org/updates/csound-6.03.2-1.fc21
Package csound-6.03.2-1.fc21: * should fix your issue, * was pushed to the Fedora 21 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing csound-6.03.2-1.fc21' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2014-10933/csound-6.03.2-1.fc21 then log in and leave karma (feedback).
csound-6.03.2-1.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.