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

fusl/src/multibyte/wcsrtombs.c

Index: fusl/src/multibyte/wcsrtombs.c
diff --git a/fusl/src/multibyte/wcsrtombs.c b/fusl/src/multibyte/wcsrtombs.c
new file mode 100644
index 0000000000000000000000000000000000000000..30be415d0683f287bf575a5b33e8c493f9b42434
--- /dev/null
+++ b/fusl/src/multibyte/wcsrtombs.c
@@ -0,0 +1,61 @@
+/* 
+ * 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 wcsrtombs(char *restrict s, const wchar_t **restrict ws, size_t n, mbstate_t *restrict st)
+{
+	const wchar_t *ws2;
+	char buf[4];
+	size_t N = n, l;
+	if (!s) {
+		for (n=0, ws2=*ws; *ws2; ws2++) {
+			if (*ws2 >= 0x80u) {
+				l = wcrtomb(buf, *ws2, 0);
+				if (!(l+1)) return -1;
+				n += l;
+			} else n++;
+		}
+		return n;
+	}
+	while (n>=4) {
+		if (**ws-1u >= 0x7fu) {
+			if (!**ws) {
+				*s = 0;
+				*ws = 0;
+				return N-n;
+			}
+			l = wcrtomb(s, **ws, 0);
+			if (!(l+1)) return -1;
+			s += l;
+			n -= l;
+		} else {
+			*s++ = **ws;
+			n--;
+		}
+		(*ws)++;
+	}
+	while (n) {
+		if (**ws-1u >= 0x7fu) {
+			if (!**ws) {
+				*s = 0;
+				*ws = 0;
+				return N-n;
+			}
+			l = wcrtomb(buf, **ws, 0);
+			if (!(l+1)) return -1;
+			if (l>n) return N-n;
+			wcrtomb(s, **ws, 0);
+			s += l;
+			n -= l;
+		} else {
+			*s++ = **ws;
+			n--;
+		}
+		(*ws)++;
+	}
+	return N;
+}