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

fusl/src/thread/pthread_mutexattr_settype.c

Index: fusl/src/thread/pthread_mutexattr_settype.c
diff --git a/fusl/src/thread/pthread_mutexattr_settype.c b/fusl/src/thread/pthread_mutexattr_settype.c
new file mode 100644
index 0000000000000000000000000000000000000000..cd7a80e342c69981f6f3095e17dfb88739c38281
--- /dev/null
+++ b/fusl/src/thread/pthread_mutexattr_settype.c
@@ -0,0 +1,8 @@
+#include "pthread_impl.h"
+
+int pthread_mutexattr_settype(pthread_mutexattr_t *a, int type)
+{
+	if ((unsigned)type > 2) return EINVAL;
+	a->__attr = (a->__attr & ~3) | type;
+	return 0;
+}