Rename include guards for consistency and to avoid reserved identifiers

The C99 spec states in section 7.1.3 ("Reserved identifiers"):

> All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
> ...
> If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.