diff --git a/compat/posix.h b/compat/posix.h
index 245386fa4a..6a137480d9 100644
--- a/compat/posix.h
+++ b/compat/posix.h
@@ -70,7 +70,9 @@
 #define _ALL_SOURCE 1
 #define _GNU_SOURCE 1
 #define _BSD_SOURCE 1
+#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE 1
+#endif
 #define _NETBSD_SOURCE 1
 #define _SGI_SOURCE 1