Make use of -lppapi explicit to allow for correct library order.

native_client_sdk/src/libraries/pthread/pthread.h

+/* Copyright (c) 2012 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef LIBRARIES_PTHREAD_PTHREAD_H_
+#define LIBRARIES_PTHREAD_PTHREAD_H_
+
+/**
+* Implementation of pthread.h for building the SDK natively on Windows.
+*/
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef int pthread_mutex_t;
+
+int pthread_mutex_init(pthread_mutex_t* m, void* traits);
+int pthread_mutex_destroy(pthread_mutex_t* m);
+
+int pthread_mutex_lock(pthread_mutex_t* m);
+int pthread_mutex_unlock(pthread_mutex_t* m);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  /* LIBRARIES_PTHREAD_PTHREAD_H_ */