Add priority_mutex and priority_condition_variable. (#50)

Also provide wpi::mutex and wpi::condition_variable as wrappers for these
on Linux (where they're available), and for std::mutex and
std::condition_variable on other platforms.

src/main/native/include/support/condition_variable.h

@@ -0,0 +1,22 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <condition_variable>
+
+#include "priority_condition_variable.h"
+
+namespace wpi {
+
+#ifdef WPI_HAVE_PRIORITY_CONDITION_VARIABLE
+using condition_variable = priority_condition_variable;
+#else
+using condition_variable = ::std::condition_variable;
+#endif
+
+}  // namespace wpi