Remove the EXTERN_C marco

[LaszloLango]

Related discussion: #873 (comment)

Use the block style instead of define EXTERN_C marco. It's much better practice to have the #ifdef __cplusplus at the very beginning of the headers. Prefer to have the order

/**
  * Copyright ...
  */

#ifndef HEADERNAME_H
#define HEADERNAME_H

#include ...

#ifdef _cplusplus
extern "C" {
#endif /* !__cplusplus */
all header content (including typedes and all, not only function declarations)
#ifdef _cplusplus
}
#endif /* !__cplusplus */
#endif // HEADERNAME_H