Use function format attribute where applicable

Allow the compiler to verify the format string against the supplied
arguments.

    chage.c:239:51: warning: format not a string literal, format string not checked [-Wformat-nonliteral]
      239 |                 (void) strftime (buf, sizeof buf, format, tp);
          |                                                   ^~~~~~

lib/defines.h

@@ -320,8 +320,10 @@ extern char *strerror ();
 /* To be used for verified unused parameters */
 #if defined(__GNUC__) && !defined(__STRICT_ANSI__)
 # define unused __attribute__((unused))
+# define format_attr(type, index, check) __attribute__((format (type, index, check)))
 #else
 # define unused
+# define format_attr(type, index, check)
 #endif
 
 /* ! Arguments evaluated twice ! */

lib/selinux.c

@@ -109,7 +109,7 @@ int reset_selinux_file_context (void)
 /*
  *  Log callback for libselinux internal error reporting.
  */
-__attribute__((__format__ (printf, 2, 3)))
+format_attr(printf, 2, 3)
 static int selinux_log_cb (int type, const char *fmt, ...) {
 	va_list ap;
 	char *buf;

lib/semanage.c

@@ -27,6 +27,7 @@
 #endif
 
 
+format_attr(printf, 3, 4)
 static void semanage_error_callback (unused void *varg,
                                      semanage_handle_t *handle,
                                      const char *fmt, ...)