Using underscore prefixes is not allowed for include guards as all
defines starting with underscore are reserved. At the same time made
file ends and spacing and style of include guards consistent in all
headers.
The new style of include guards also includes the directory of the
include file relative to `src` directory to prevent potential future
issues where we might end up with a name clash across directories.
Rafael Silva
Piotr Esden-Tempski
Gareth McMullin