Use #pragma once

[matthew-reynolds]

Part of #403.

Replace all preprocessor header guards with #pragma once. Also adds missing header guard to transmission_interface/test/loader_utils.h. Every .h file now includes #pragma once.

[Karsten1987]

I just wanted to mention here that this pragma is not standard compliant. As most compilers do support it, but as said it’s not in the official standard.
https://stackoverflow.com/questions/23696115/is-pragma-once-part-of-the-c11-standard#23699893

[bmagyar]

If all relevant compilers support it (heck, even MSVC does!) then I think it is a safe bet.

[matthew-reynolds]

Rebased and updated following #395

[bmagyar]

Looks great, thanks a lot!

[mathias-luedtke]

Ah, I am too late for this one.

If all relevant compilers support it (heck, even MSVC does!) then I think it is a safe bet.

Not all versions support it and some implementations have serious flaws.
Even worse: If a compiler does not support it, it must (?) ignore it.

[mathias-luedtke]

I would stick to the guidelines: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#sf8-use-include-guards-for-all-h-files

[matthew-reynolds]

I don't have a strong opinion here, I'm just following #403. Happy to revert to #ifndef guards if that's the consensus of you maintainers.

I do wonder how many C++14-compatible compilers don't support #pragma once, but that's not an easy audit to make to cover every obscure compiler out there.