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

fusl/src/thread/mtx_timedlock.c

Index: fusl/src/thread/mtx_timedlock.c
diff --git a/fusl/src/thread/mtx_timedlock.c b/fusl/src/thread/mtx_timedlock.c
new file mode 100644
index 0000000000000000000000000000000000000000..bcc152c564348898c1ed9218aebb3dc3776a7c23
--- /dev/null
+++ b/fusl/src/thread/mtx_timedlock.c
@@ -0,0 +1,14 @@
+#include <threads.h>
+#include <errno.h>
+
+int __pthread_mutex_timedlock(mtx_t *restrict, const struct timespec *restrict);
+
+int mtx_timedlock(mtx_t *restrict m, const struct timespec *restrict ts)
+{
+	int ret = __pthread_mutex_timedlock(m, ts);
+	switch (ret) {
+	default:        return thrd_error;
+	case 0:         return thrd_success;
+	case ETIMEDOUT: return thrd_timedout;
+	}
+}