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

fusl/src/multibyte/wcrtomb.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 <stdlib.h>
+#include <wchar.h>
+#include <errno.h>
+#include "internal.h"
+
+size_t wcrtomb(char *restrict s, wchar_t wc, mbstate_t *restrict st)
+{
+        if (!s) return 1;
+        if ((unsigned)wc < 0x80) {
+                *s = wc;
+                return 1;
+        } else if (MB_CUR_MAX == 1) {
+                if (!IS_CODEUNIT(wc)) {
+                        errno = EILSEQ;
+                        return -1;
+                }
+                *s = wc;
+                return 1;
+        } else if ((unsigned)wc < 0x800) {
+                *s++ = 0xc0 | (wc>>6);
+                *s = 0x80 | (wc&0x3f);
+                return 2;
+        } else if ((unsigned)wc < 0xd800 || (unsigned)wc-0xe000 < 0x2000) {
+                *s++ = 0xe0 | (wc>>12);
+                *s++ = 0x80 | ((wc>>6)&0x3f);
+                *s = 0x80 | (wc&0x3f);
+                return 3;
+        } else if ((unsigned)wc-0x10000 < 0x100000) {
+                *s++ = 0xf0 | (wc>>18);
+                *s++ = 0x80 | ((wc>>12)&0x3f);
+                *s++ = 0x80 | ((wc>>6)&0x3f);
+                *s = 0x80 | (wc&0x3f);
+                return 4;
+        }
+        errno = EILSEQ;
+        return -1;
+}