diff --git a/tools/re/main.c b/tools/re/main.c
index 9bc5cdf..b512ad8 100644
--- a/tools/re/main.c
+++ b/tools/re/main.c
@@ -12,8 +12,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
-#define _GNU_SOURCE
-#define _POSIX_C_SOURCE 200809L /* strdup, getline */
+#define _GNU_SOURCE /* asprintf, strdup */
+#include <sys/cdefs.h>
 
 #include <errno.h>
 #include <getopt.h>