Double-include guard

[Michi Henning]

We recently change our double-include guards to use

#pragma once

That's a lot briefer than the usual #ifndef ... #define ... #endif guard. It also is safer: I've more than once made the mistake of copying a header file to create new similar header and forgetting to update the guard symbol. When things don't compile, it can be quite difficult to find which header blocks inclusion of some other header because they both use the same guard symbol...

The pragma works for gcc, clang (and even Visual C++), which are pretty much the only compilers that matter these days, except maybe for Visual C++ ;-)

I think we should update the guide accordingly.