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

fusl/src/stdio/ungetwc.c

+#include "stdio_impl.h"
+#include "locale_impl.h"
+#include <wchar.h>
+#include <limits.h>
+#include <ctype.h>
+#include <string.h>
+
+wint_t ungetwc(wint_t c, FILE *f)
+{
+        unsigned char mbc[MB_LEN_MAX];
+        int l=1;
+        locale_t *ploc = &CURRENT_LOCALE, loc = *ploc;
+
+        FLOCK(f);
+
+        if (f->mode <= 0) fwide(f, 1);
+        *ploc = f->locale;
+
+        if (!f->rpos) __toread(f);
+        if (!f->rpos || f->rpos < f->buf - UNGET + l || c == WEOF ||
+            (!isascii(c) && (l = wctomb((void *)mbc, c)) < 0)) {
+                FUNLOCK(f);
+                *ploc = loc;
+                return WEOF;
+        }
+
+        if (isascii(c)) *--f->rpos = c;
+        else memcpy(f->rpos -= l, mbc, l);
+
+        f->flags &= ~F_EOF;
+
+        FUNLOCK(f);
+        *ploc = loc;
+        return c;
+}