diff --git a/config.mk b/config.mk
index 9db2d96..0970abf 100644
--- a/config.mk
+++ b/config.mk
@@ -8,6 +8,6 @@ MANPREFIX = $(PREFIX)/share/man
 #CC = gcc
 #CC = musl-gcc
 LD = $(CC)
-CPPFLAGS = -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
+CPPFLAGS = -D_XOPEN_SOURCE -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
 CFLAGS   = -std=c99 -Wall -Wextra $(CPPFLAGS)
 LDFLAGS  = -s -lcrypt # -static
diff --git a/login.c b/login.c
index bc90818..fed0945 100644
--- a/login.c
+++ b/login.c
@@ -1,5 +1,4 @@
 /* See LICENSE file for copyright and license details. */
-#define _XOPEN_SOURCE
 #include <errno.h>
 #include <grp.h>
 #include <pwd.h>
diff --git a/su.c b/su.c
index 47d3fe7..63e70ca 100644
--- a/su.c
+++ b/su.c
@@ -1,5 +1,4 @@
 /* See LICENSE file for copyright and license details. */
-#define _XOPEN_SOURCE
 #include <sys/types.h>
 #include <unistd.h>
 #include <errno.h>