Add a "fork" of musl as //fusl.

fusl/src/multibyte/mblen.c

Index: fusl/src/multibyte/mblen.c
diff --git a/fusl/src/multibyte/mblen.c b/fusl/src/multibyte/mblen.c
new file mode 100644
index 0000000000000000000000000000000000000000..96b47b123842ba9810a63d7617a2c1c199fe7b7f
--- /dev/null
+++ b/fusl/src/multibyte/mblen.c
@@ -0,0 +1,12 @@
+/* 
+ * This code was written by Rich Felker in 2010; no copyright is claimed.
+ * This code is in the public domain. Attribution is appreciated but
+ * unnecessary.
+ */
+
+#include <stdlib.h>
+
+int mblen(const char *s, size_t n)
+{
+	return mbtowc(0, s, n);
+}