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

fusl/src/termios/cfsetospeed.c

+#include <termios.h>
+#include <sys/ioctl.h>
+#include <errno.h>
+#include "libc.h"
+
+int cfsetospeed(struct termios *tio, speed_t speed)
+{
+        if (speed & ~CBAUD) {
+                errno = EINVAL;
+                return -1;
+        }
+        tio->c_cflag &= ~CBAUD;
+        tio->c_cflag |= speed;
+        return 0;
+}
+
+int cfsetispeed(struct termios *tio, speed_t speed)
+{
+        return speed ? cfsetospeed(tio, speed) : 0;
+}
+
+weak_alias(cfsetospeed, cfsetspeed);