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

fusl/src/linux/timerfd.c

+#include <sys/timerfd.h>
+#include "syscall.h"
+
+int timerfd_create(int clockid, int flags)
+{
+        return syscall(SYS_timerfd_create, clockid, flags);
+}
+
+int timerfd_settime(int fd, int flags, const struct itimerspec *new, struct itimerspec *old)
+{
+        return syscall(SYS_timerfd_settime, fd, flags, new, old);
+}
+
+int timerfd_gettime(int fd, struct itimerspec *cur)
+{
+        return syscall(SYS_timerfd_gettime, fd, cur);
+}