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

fusl/src/thread/pthread_setcancelstate.c

Index: fusl/src/thread/pthread_setcancelstate.c
diff --git a/fusl/src/thread/pthread_setcancelstate.c b/fusl/src/thread/pthread_setcancelstate.c
new file mode 100644
index 0000000000000000000000000000000000000000..5ab8c338f7969c8d79d24fe1a9c18a7481e62555
--- /dev/null
+++ b/fusl/src/thread/pthread_setcancelstate.c
@@ -0,0 +1,12 @@
+#include "pthread_impl.h"
+
+int __pthread_setcancelstate(int new, int *old)
+{
+	if (new > 2U) return EINVAL;
+	struct pthread *self = __pthread_self();
+	if (old) *old = self->canceldisable;
+	self->canceldisable = new;
+	return 0;
+}
+
+weak_alias(__pthread_setcancelstate, pthread_setcancelstate);