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

fusl/src/multibyte/mbtowc.c

Index: fusl/src/multibyte/mbtowc.c
diff --git a/fusl/src/multibyte/mbtowc.c b/fusl/src/multibyte/mbtowc.c
new file mode 100644
index 0000000000000000000000000000000000000000..71a950666c8cef7375d92572894fd1054b75d4ff
--- /dev/null
+++ b/fusl/src/multibyte/mbtowc.c
@@ -0,0 +1,53 @@
+/* 
+ * 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"
+
+int mbtowc(wchar_t *restrict wc, const char *restrict src, size_t n)
+{
+	unsigned c;
+	const unsigned char *s = (const void *)src;
+	wchar_t dummy;
+
+	if (!s) return 0;
+	if (!n) goto ilseq;
+	if (!wc) wc = &dummy;
+
+	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];
+
+	/* Avoid excessive checks against n: If shifting the state n-1
+	 * times does not clear the high bit, then the value of n is
+	 * insufficient to read a character */
+	if (n<4 && ((c<<(6*n-6)) & (1U<<31))) goto ilseq;
+
+	if (OOB(c,*s)) goto ilseq;
+	c = c<<6 | *s++-0x80;
+	if (!(c&(1U<<31))) {
+		*wc = c;
+		return 2;
+	}
+
+	if (*s-0x80u >= 0x40) goto ilseq;
+	c = c<<6 | *s++-0x80;
+	if (!(c&(1U<<31))) {
+		*wc = c;
+		return 3;
+	}
+
+	if (*s-0x80u >= 0x40) goto ilseq;
+	*wc = c<<6 | *s++-0x80;
+	return 4;
+
+ilseq:
+	errno = EILSEQ;
+	return -1;
+}