Is it possible to hook up macbook build in keyboard to Ubuntu PC? I need to work on both: Mac Pro and Ubuntu PC.

I would like to use one keyboard. It is more less simple to do with external Mac keyboard, but how to do it with build-in one?

External is not convenient, because I use 3 or 4 screens (including laptop one) and the whole 'work env' gets either too wide or too deep.
Have you tried Synergy? They say that it  lets you easily share your mouse and keyboard between multiple computers on your desktop.

There are some videos on Youtube which show how it works.