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

fusl/include/uchar.h

+#ifndef _UCHAR_H
+#define _UCHAR_H
+
+#ifdef __cplusplus
+extern "C" {
+#else
+typedef unsigned short char16_t;
+typedef unsigned char32_t;
+#endif
+
+#define __NEED_mbstate_t
+#define __NEED_size_t
+
+#include <features.h>
+#include <bits/alltypes.h>
+
+size_t c16rtomb(char *__restrict, char16_t, mbstate_t *__restrict);
+size_t mbrtoc16(char16_t *__restrict, const char *__restrict, size_t, mbstate_t *__restrict);
+
+size_t c32rtomb(char *__restrict, char32_t, mbstate_t *__restrict);
+size_t mbrtoc32(char32_t *__restrict, const char *__restrict, size_t, mbstate_t *__restrict);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif