Migrate include guards to #pragma once from #define 

Due to opening for external libraries, possible future code restructuring in order to avoid possible name clashes, need for maintaining coherent naming scheme, reduce code overhead I open request for migrating include guards from 

#ifndef FILE_NAME_H
#define FILE_NAME_H
...
#endif

to

#pragma once

The construct is not standarized by ISO, but de-factor mature industry standard.
For unsupporting compilers the construct can be easily substituted.