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

fusl/src/thread/cnd_signal.c

Index: fusl/src/thread/cnd_signal.c
diff --git a/fusl/src/thread/cnd_signal.c b/fusl/src/thread/cnd_signal.c
new file mode 100644
index 0000000000000000000000000000000000000000..1211260b4b89548476314245bd282330332f96c6
--- /dev/null
+++ b/fusl/src/thread/cnd_signal.c
@@ -0,0 +1,10 @@
+#include <threads.h>
+
+int __private_cond_signal(cnd_t *, int);
+
+int cnd_signal(cnd_t *c)
+{
+	/* This internal function never fails, and always returns zero,
+	 * which matches the value thrd_success is defined with. */
+	return __private_cond_signal(c, 1);
+}