--- a/usr.bin/doas/doas.c	2020-10-16 13:11:26.769202572 +0300
+++ b/usr.bin/doas/doas.c	2020-10-16 13:09:43.558056917 +0300
@@ -15,6 +15,10 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>