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

fusl/src/multibyte/mbrlen.c

+/* 
+ * 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 <wchar.h>
+
+size_t mbrlen(const char *restrict s, size_t n, mbstate_t *restrict st)
+{
+        static unsigned internal;
+        return mbrtowc(0, s, n, st ? st : (mbstate_t *)&internal);
+}