Swap from include guards to 'pragma once'.

From a purely standard-compliance perspective this is a bad idea.
However:

 - The code base is written completely with Visual Studio's compiler in
   mind;

 - There are already pragma pre-processor statements that are specific to
   Visual Studios compiler anyway;

 - A large number of compilers support pragma once anyway; and

 - It may improve compile time in the long run.

And from a really nit-picky point of view not using include guards
prevents the IDE explaining that there is a random definition of a
constant in the object browser.