diff --git a/m4/crypt_xpg6.m4 b/m4/crypt_xpg6.m4
index 0085b2ac769..3a288a3713a 100644
--- a/m4/crypt_xpg6.m4
+++ b/m4/crypt_xpg6.m4
@@ -6,6 +6,7 @@ AC_DEFUN([DOVECOT_CRYPT_XPG6], [
       #define _XOPEN_SOURCE 4
       #define _XOPEN_SOURCE_EXTENDED 1
       #define _XOPEN_VERSION 4
+      #define _DEFAULT_SOURCE
       #define _XPG4_2
       #define _XPG6
       #include <unistd.h>