Barf instead of trying to fix things if __KLIBC__ not defined.  It's
a serious error and needs to be caught.

diff --git a/include/stddef.h b/include/stddef.h
index 125d235..e96f28c 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -6,7 +6,7 @@
 #define _STDDEF_H
 
 #ifndef __KLIBC__
-# define __KLIBC__ 1
+# error "__KLIBC__ not defined, compiler invocation error!"
 #endif
 
 #include <bitsize/stddef.h>