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

fusl/src/stdio/ungetwc.c

Index: fusl/src/stdio/ungetwc.c
diff --git a/fusl/src/stdio/ungetwc.c b/fusl/src/stdio/ungetwc.c
new file mode 100644
index 0000000000000000000000000000000000000000..80d6e203d0765564a21e016f8be9577f6a3c67a2
--- /dev/null
+++ b/fusl/src/stdio/ungetwc.c
@@ -0,0 +1,35 @@
+#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;
+}