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 wasn't sure what CEF version you were using so I used the most recent release (1738).
```include/cef_version.h``` declares ```CEF_REVISION``` so I'd rather do something like this:
```C
#if CEF_REVISION < 1739
    CefRect client_popup_rect(0, 0, mPopupRect.width, mPopupRect.height);
    browser->GetHost()->Invalidate(client_popup_rect, PET_POPUP);
#else
    browser->GetHost()->Invalidate(PET_POPUP);
#endif
```

Do you know which revision introduced the change? I'll have to do the same for ```info.SetAsWindowless()```.