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.
Maybe you could make the coordinates and size arguments optional and default to (0,0) for the position and the screen size for the size arguments so a script could just do `ch.createBrowser()` to create a full screen browser.