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

fusl/src/thread/mtx_trylock.c

Index: fusl/src/thread/mtx_trylock.c
diff --git a/fusl/src/thread/mtx_trylock.c b/fusl/src/thread/mtx_trylock.c
new file mode 100644
index 0000000000000000000000000000000000000000..61e7694edcda2528f8c184a4e91feb7060b23051
--- /dev/null
+++ b/fusl/src/thread/mtx_trylock.c
@@ -0,0 +1,17 @@
+#include "pthread_impl.h"
+#include <threads.h>
+
+int __pthread_mutex_trylock(mtx_t *);
+
+int mtx_trylock(mtx_t *m)
+{
+	if (m->_m_type == PTHREAD_MUTEX_NORMAL)
+		return (a_cas(&m->_m_lock, 0, EBUSY) & EBUSY) ? thrd_busy : thrd_success;
+
+	int ret = __pthread_mutex_trylock(m);
+	switch (ret) {
+	default:    return thrd_error;
+	case 0:     return thrd_success;
+	case EBUSY: return thrd_busy;
+	}
+}