Move common utility classes to wpiutil library. (#79)

This is a breaking change to dependencies that use the static ntcore
library.  Unless the wpiutil library is also linked, linker errors will
result.  This does not affect the shared ntcore library.

wpiutil/include/support/atomic_static.h

@@ -0,0 +1,49 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_SUPPORT_ATOMIC_STATIC_H_
+#define WPIUTIL_SUPPORT_ATOMIC_STATIC_H_
+
+#if !defined(_MSC_VER) || (_MSC_VER >= 1900)
+
+// Just use a local static.  This is thread-safe per
+// http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/
+
+// Per https://msdn.microsoft.com/en-us/library/Hh567368.aspx "Magic Statics"
+// are supported in Visual Studio 2015 but not in earlier versions.
+#define ATOMIC_STATIC(cls, inst) static cls inst
+#define ATOMIC_STATIC_DECL(cls)
+#define ATOMIC_STATIC_INIT(cls)
+
+#else
+// From http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/
+#include <atomic>
+#include <mutex>
+
+#define ATOMIC_STATIC(cls, inst) \
+    cls* inst##tmp = m_instance.load(std::memory_order_acquire); \
+    if (inst##tmp == nullptr) { \
+      std::lock_guard<std::mutex> lock(m_instance_mutex); \
+      inst##tmp = m_instance.load(std::memory_order_relaxed); \
+      if (inst##tmp == nullptr) { \
+        inst##tmp = new cls; \
+        m_instance.store(inst##tmp, std::memory_order_release); \
+      } \
+    } \
+    cls& inst = *inst##tmp
+
+#define ATOMIC_STATIC_DECL(cls) \
+    static std::atomic<cls*> m_instance; \
+    static std::mutex m_instance_mutex;
+
+#define ATOMIC_STATIC_INIT(cls) \
+    std::atomic<cls*> cls::m_instance; \
+    std::mutex cls::m_instance_mutex;
+
+#endif
+
+#endif  // WPIUTIL_SUPPORT_ATOMIC_STATIC_H_