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

fusl/src/thread/pthread_setcanceltype.c

Index: fusl/src/thread/pthread_setcanceltype.c
diff --git a/fusl/src/thread/pthread_setcanceltype.c b/fusl/src/thread/pthread_setcanceltype.c
new file mode 100644
index 0000000000000000000000000000000000000000..bf0a3f383dd34943c6eb502f50043b4dddcff537
--- /dev/null
+++ b/fusl/src/thread/pthread_setcanceltype.c
@@ -0,0 +1,11 @@
+#include "pthread_impl.h"
+
+int pthread_setcanceltype(int new, int *old)
+{
+	struct pthread *self = __pthread_self();
+	if (new > 1U) return EINVAL;
+	if (old) *old = self->cancelasync;
+	self->cancelasync = new;
+	if (new) pthread_testcancel();
+	return 0;
+}