diff --git a/include/libmilter/mfapi.h b/include/libmilter/mfapi.h
index 8209882c7..61236d5bb 100644
--- a/include/libmilter/mfapi.h
+++ b/include/libmilter/mfapi.h
@@ -89,6 +89,21 @@ typedef int	sfsistat;
 # endif
 #endif /* __P */
 
+/*
+**  SM_CONF_STDBOOL_H is 1 if <stdbool.h> exists
+**
+**  Note, unlike gcc, clang doesn't apply full prototypes to K&R definitions.
+*/
+
+# ifndef SM_CONF_STDBOOL_H
+#  if !defined(__clang__) && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+#   define SM_CONF_STDBOOL_H		1
+#  else
+#   define SM_CONF_STDBOOL_H		0
+#  endif
+# endif /* ! SM_CONF_STDBOOL_H */
+
+
 #if SM_CONF_STDBOOL_H
 # include <stdbool.h>
 #else /* SM_CONF_STDBOOL_H */