glibc/include/libintl.h

#ifndef _LIBINTL_H
#include <intl/libintl.h>

/* Now define the internal interfaces.  */
extern char *__gettext __P ((__const char *__msgid));
extern char *__textdomain __P ((__const char *__domainname));
extern char *__bindtextdomain __P ((__const char *__domainname,
				    __const char *__dirname));
extern const char _libc_intl_domainname[];

/* Define the macros `_' and `N_' for conveniently marking translatable
   strings in the libc source code.  */

# define N_(msgid)	msgid

# ifdef dgettext
/* This is defined as an optimizing macro, so use it.  */
#  define _(msgid)	dgettext (_libc_intl_domainname, (msgid))
# else
/* Be sure to use only the __ name when `dgettext' is a plain function
   instead of an optimizing macro.  */
#  define _(msgid)	__dgettext (_libc_intl_domainname, (msgid))
# endif

#endif