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.

This isn't finished yet, right now [Boost fails to compile](https://gist.github.com/ngld/25248cab5411214df7f0) since it doesn't like SCP's ```strcat_s()``` and ```strcpy_s()```.
You should use `target_include_directories(${NAME} PUBLIC "${CEF_PATH}")` here so the dependent targets also include this.