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.
The point was that if you use a wrong path in browserHandle:create() or in states.tbl you don't get any warning or error which can be very confusing.
I'd suggest moving this to a better place but I don't know where to put this...