add format argument attributes to gettext function prototypes

their absence completely breaks format string warnings in programs
with gettext message translations: -Wformat gives no results, and
-Wformat-nonliteral produces spurious warnings.

with gcc, the problem manifests only in standards-conforming profiles;
otherwise gcc sets these attributes by default for the gettext family.
with clang, the problem always manifests; clang has no such defaults.

include/libintl.h

@@ -8,16 +8,24 @@ extern "C" {
 #define __USE_GNU_GETTEXT 1
 #define __GNU_GETTEXT_SUPPORTED_REVISION(major) ((major) == 0 ? 1 : -1)
 
-char *gettext(const char *);
-char *dgettext(const char *, const char *);
-char *dcgettext(const char *, const char *, int);
-char *ngettext(const char *, const char *, unsigned long);
-char *dngettext(const char *, const char *, const char *, unsigned long);
-char *dcngettext(const char *, const char *, const char *, unsigned long, int);
+#if __GNUC__ >= 3
+#define __fa(n) __attribute__ ((__format_arg__ (n)))
+#else
+#define __fa(n)
+#endif
+
+char *gettext(const char *) __fa(1);
+char *dgettext(const char *, const char *) __fa(2);
+char *dcgettext(const char *, const char *, int) __fa(2);
+char *ngettext(const char *, const char *, unsigned long) __fa(1) __fa(2);
+char *dngettext(const char *, const char *, const char *, unsigned long) __fa(2) __fa(3);
+char *dcngettext(const char *, const char *, const char *, unsigned long, int) __fa(2) __fa(3);
 char *textdomain(const char *);
 char *bindtextdomain (const char *, const char *);
 char *bind_textdomain_codeset(const char *, const char *);
 
+#undef __fa
+
 #ifdef __cplusplus
 }
 #endif