Just incase a third stab at it helps (and I'm sort of crossing into SO territority, but...) :
Applying the patch/options/etc (to enable real-time support) will only
have a direct impact on the kernel.
Software written in userspace
will see flow-on impacts, but they will not be things like changes in
the API, different return values, etc etc etc (basically - what @LaC
said). They will instead be things like 'better' real-time
performance because the userspace software is interfacing with something that has been set to operate in a more real-time fashion.
As a slight digression, I'd caveat the 'better' performance bit - you're not going to get true real-time operation from anything unless the whole system is designed for real-time operation (as mentioned in the SO response).. in fact, now I think about it, you could reword it to talk about the intent of the statement as follows:
If you enable these patches/options, it will make the kernel operate in a real-time mode.
This does not make your userspace programming experience any different.
You program the same way. You compile the same way. You can do everything the same as you did before. The only thing you might see, is better real-time behaviour.