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

fusl/src/multibyte/mbrlen.c

Index: fusl/src/multibyte/mbrlen.c
diff --git a/fusl/src/multibyte/mbrlen.c b/fusl/src/multibyte/mbrlen.c
new file mode 100644
index 0000000000000000000000000000000000000000..3a5a74336287fc71d167b30433911b3e3d66b009
--- /dev/null
+++ b/fusl/src/multibyte/mbrlen.c
@@ -0,0 +1,13 @@
+/* 
+ * 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);
+}