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()```.
I'm passing Cmdline_argc and Cmdline_argv to [CefMainArgs](https://github.com/asarium/fs2open.github.com/pull/1/files#diff-ca25488e9dffcd9de5b3d44cb0bf4dfaR79).
Why would this cause issues? I did it this way so I can put stuff like ```--disable-setuid-sandbox``` in my ```cmdline_fso.cfg```.