conditionally build strnlen if needed.

it is only used by strndup

prodded by Sortie@

include/string.h

@@ -22,8 +22,10 @@ size_t strlcat(char *dst, const char *src, size_t siz);
 
 #ifdef NO_STRNDUP
 char * strndup(const char *str, size_t maxlen);
+#ifdef NO_STRNLEN
 size_t strnlen(const char *str, size_t maxlen);
 #endif
+#endif
 
 #ifdef NO_EXPLICIT_BZERO
 void explicit_bzero(void *, size_t);