Make it possible to compile this on Linux

The changes to cmake/util.cmake aren't mine, I simply copied the file from the cmake branch.
I added GTK as a dependency for the CEF headers and made some small changes to make gcc happy.
That is why you can pass a weight to `os::addEventListener()`. By passing something lower than `os::DEFAULT_LISTENER_WEIGHT` you can make sure that the listener is executed before the default listener. With this solution you don't know if the event actually got handled by the default handler if you want to intercept events not handled by it.