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

fusl/src/multibyte/mbrtowc.c

Index: fusl/src/multibyte/mbrtowc.c
diff --git a/fusl/src/multibyte/mbrtowc.c b/fusl/src/multibyte/mbrtowc.c
new file mode 100644
index 0000000000000000000000000000000000000000..ca7da700ed7f0ec7d4b34d982625b8cb7bef0ee5
--- /dev/null
+++ b/fusl/src/multibyte/mbrtowc.c
@@ -0,0 +1,57 @@
+/* 
+ * 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>
+#include <wchar.h>
+#include <errno.h>
+#include "internal.h"
+
+size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate_t *restrict st)
+{
+	static unsigned internal_state;
+	unsigned c;
+	const unsigned char *s = (const void *)src;
+	const unsigned N = n;
+	wchar_t dummy;
+
+	if (!st) st = (void *)&internal_state;
+	c = *(unsigned *)st;
+	
+	if (!s) {
+		if (c) goto ilseq;
+		return 0;
+	} else if (!wc) wc = &dummy;
+
+	if (!n) return -2;
+	if (!c) {
+		if (*s < 0x80) return !!(*wc = *s);
+		if (MB_CUR_MAX==1) return (*wc = CODEUNIT(*s)), 1;
+		if (*s-SA > SB-SA) goto ilseq;
+		c = bittab[*s++-SA]; n--;
+	}
+
+	if (n) {
+		if (OOB(c,*s)) goto ilseq;
+loop:
+		c = c<<6 | *s++-0x80; n--;
+		if (!(c&(1U<<31))) {
+			*(unsigned *)st = 0;
+			*wc = c;
+			return N-n;
+		}
+		if (n) {
+			if (*s-0x80u >= 0x40) goto ilseq;
+			goto loop;
+		}
+	}
+
+	*(unsigned *)st = c;
+	return -2;
+ilseq:
+	*(unsigned *)st = 0;
+	errno = EILSEQ;
+	return -1;
+}