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

fusl/src/stdio/setvbuf.c

+#include "stdio_impl.h"
+
+/* This function makes no attempt to protect the user from his/her own
+ * stupidity. If called any time but when then ISO C standard specifically
+ * allows it, all hell can and will break loose, especially with threads!
+ *
+ * This implementation ignores all arguments except the buffering type,
+ * and uses the existing buffer allocated alongside the FILE object.
+ * In the case of stderr where the preexisting buffer is length 1, it
+ * is not possible to set line buffering or full buffering. */
+
+int setvbuf(FILE *restrict f, char *restrict buf, int type, size_t size)
+{
+        f->lbf = EOF;
+
+        if (type == _IONBF)
+                f->buf_size = 0;
+        else if (type == _IOLBF)
+                f->lbf = '\n';
+
+        f->flags |= F_SVB;
+
+        return 0;
+}