Reserved Identifiers Used In Header Guards

[insertinterestingnamehere]

We shouldn't be using identifiers structured like _QT_HEADERNAME_H_ since these are reserved identifiers in the C standard. I'm seeing these kinds of things in the header include guards, but there are probably more examples floating around.

This more of a language lawyer thing though since current compilers do let this kind of thing slide. Because of this, it's low-priority right now.