These are not C functions but macros.

In SWI-Prolog.h, we find:

#ifndef TRUE
#define TRUE	(1)
#define FALSE	(0)
#endif

#define	PL_succeed	return TRUE	/* succeed deterministically */
#define PL_fail		return FALSE	/* fail */

So using TRUE and FALSE as "Prolog TRUE" and "Prolog FALSE" elsewhere seems appropriate.

On the C level, there is stdbool.h:

#define bool	_Bool
#define true	1
#define false	0

which makes bool compatible (not really surprisingly).