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 file needs the header because it uses ```std::for_each()```. [Line 909](https://github.com/ngld/fs2open/blob/feature/chromiumInput/code/menuui/barracks.cpp#L908)
```C
// Remove file ectensions
std::for_each(fileNames.begin(), fileNames.end(), cfile::util::removeExtension<SCP_string>);
```
See: http://en.cppreference.com/w/cpp/header/algorithm