Fix build failure on Mac OS 10.6.8

This fix disables SecRandomCopyBytes in OSX <10.7, to [allow it to build](https://github.com/cjdelisle/cjdns/issues/199) in those environments.

SecRandomCopyBytes was introduced in OSX 10.7 [Lion][1] and in iOS.
It is useful on iOS since /dev/random is outside the sandbox. According to the [API docs][2], it just reads from /dev/random, so it should be okay for Snow Leopard to do without it.

[1]: http://developer.apple.com/library/mac/#releasenotes/General/MacOSXLionAPIDiffs/Security.html
[2]: http://developer.apple.com/library/ios/#documentation/Security/Reference/RandomizationReference/Reference/reference.html