Define JNIEXPORT as visibility default with GCC?

[David DeHaven]

> I did find at least a couple places in JavaFX 8.0 where it's being misused. Those should be easy to clean up though, I can file an issue and handle it. I haven't checked JavaFX 2.x yet, but that's not a concern until this gets backported.

On closer look JavaFX 8 is fine. My noisy grep output fooled me :) The headers files for (The Platform That Must Not Be Named) even set the visibility attribute to default which means a good portion of the native code has been vetted.

-DrD-