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

fusl/src/multibyte/wcsrtombs.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 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;
+}