Convert to #pragma once

[white238]

Converts all headers to #pragma once. There were a couple places that were missing guards. I also didn't touch src/thirdparty and src/cmake.

[rhornung67]

@white238 #pragma once isn't an C++ standard feature. Granted, it's cleaner than the macro guard approach, but could this cause a problem in some circumstances; e.g., sym links, moving files during the build-install process?

[kennyweiss]

Thanks @white238

This should improve consistency (as much fun as it is coming up w/ slightly inconsistent header guards for each file).

There are a few cases where the guard was a a few lines lower than it should have been. These predate your PR, but this would be a good time to fix them.

[kennyweiss]

This predates the current PR, but the #pragma once should be above the includes in this file.

[white238]

pragma once is supported by all major compilers at this point and removes the possibility of copy/paste issue. There is a possibility that if you mix symlinked and non-symlinked header it would include it and add a bit to the compile time. But personally i think its more likely we mess it up.

It is a very common feature at this point, more concise, and possibly speeds up builds.

[Arlie-Capps]

Thanks, Chris.