Define __KLIBC__ only if it's not already defined
diff --git a/include/stddef.h b/include/stddef.h
index 422652c..900c147 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -5,7 +5,9 @@
#ifndef _STDDEF_H
#define _STDDEF_H
-#define __KLIBC__ 1
+#ifndef __KLIBC__
+# define __KLIBC__ 1
+#endif
#include <bitsize/stddef.h>
diff --git a/klibc/include/stddef.h b/klibc/include/stddef.h
index 422652c..900c147 100644
--- a/klibc/include/stddef.h
+++ b/klibc/include/stddef.h
@@ -5,7 +5,9 @@
#ifndef _STDDEF_H
#define _STDDEF_H
-#define __KLIBC__ 1
+#ifndef __KLIBC__
+# define __KLIBC__ 1
+#endif
#include <bitsize/stddef.h>