[Development] GL headers in Qt5GuiConfigExtras.cmake

[Peter Kuemmel]

> 
> So, what should it *actually* be? Is it a bug that it is one value instead of 
> the other?
> 
> Where is it set to what it is set to?

I've uploaded a patch for further discussion:
https://codereview.qt-project.org/#change,78038

> 
> Thanks,
> 
> -- 
> Stephen Kelly <stephen.kelly at kdab.com> | Software Engineer
> KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
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