root: upgrade to 5.34.15 and make Cocoa backend default

This is a follow-up to #682. The only difference is that this replaces flag '--without-cocoa' with '--with-x11'. Rationale: If ROOT developers think that Cocoa backend is already good enough, then users would want X11 only when they want it for something (e.g. network transparency), and not because they don't want Cocoa, so the option name should bear a positive meaning.

I've tested both options to compile and work on OSX 10.9.1.