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.
Can you encapsulate that in a function? Something like `cmdline_get_args(int& argc_out, const char**& argv_out)` will make the code easier to read and easier to maintain.