Remove underscores from header guard

According to various sources, variable names prepended with underscores are reserved for compilers.

See [here](https://www.securecoding.cert.org/confluence/display/seccode/DCL37-C.+Do+not+declare+or+define+a+reserved+identifier#DCL37-C.Donotdeclareordefineareservedidentifier-NoncompliantCodeExample%28HeaderGuard%29) and [here](http://punchlet.wordpress.com/2009/12/01/letter-the-second/)