This is an auto-filled bug because this package does not respect user's CFLAGS. While SSP is really good for security purpose, I'd expect that with CFLAGS="-fno-stack-protector" the package should not have stack protection. In this case the file /bin/head (and maybe some other file from this package) has stack protection. To check the SSP status you can use: checksec --file /bin/head (from app-admin/checksec) hardening-check /bin/head (from app-admin/hardening-check) readelf -sW /bin/head | grep "__stack_chk_fail"
I'm sorry for the bugspam but this bug comes from a false-positive, more info at https://bugs.gentoo.org/679788#c2
