diff --git a/src/lib-mem.c b/src/lib-mem.c
index 0ba641b..2571509 100644
--- a/src/lib-mem.c
+++ b/src/lib-mem.c
@@ -39,6 +39,7 @@
 #define _DARWIN_C_SOURCE
 /* Use this to get posix_memalign */
 #if defined HAVE_POSIX_MEMALIGN
+#   undef _XOPEN_SOURCE
 #   define _XOPEN_SOURCE 600
 #endif