message: Use cross-platform std::chrono in getTickCount()
  Add conditional compilation directives to enable uses of std::chrono on supported compilers. Use std::chrono::steady_clock as a source to retrieve current tick count and clock frequency.
  Fixes opencv/opencv#6902.
diff: @@ -302,6 +302,16 @@ DECLARE_CV_CPUID_X86
  #endif
  +#if ((__cplusplus >= 201103L) || (defined(_MSC_VER) && _MSC_VER >= 1800))
  + #define HAVE_CXX11 1
  +#else
  + #define HAVE_CXX11 0
  +#endif
  +
  +#if HAVE_CXX11
  + #include <chrono>
  +#endif
  +
  namespace cv
  {
  @@ -840,7 +850,10 @@ bool useOptimized(void)
  int64 getTickCount(void)
  {
  -#if defined _WIN32 || defined WINCE
  +#if HAVE_CXX11
  + std::chrono::steady_clock::time_point now = std::chrono::steady_clock::now();
  + return (int64)now.time_since_epoch().count();
  +#elif defined _WIN32 || defined WINCE
  LARGE_INTEGER counter;
  QueryPerformanceCounter( &counter );
  return (int64)counter.QuadPart;
  @@ -859,7 +872,11 @@ int64 getTickCount(void)
  double getTickFrequency(void)
  {
  -#if defined _WIN32 || defined WINCE
  +#if HAVE_CXX11
  + using clock_period_t = std::chrono::steady_clock::duration::period;
  + double clock_freq = clock_period_t::den / clock_period_t::num;
  + return clock_freq;
  +#elif defined _WIN32 || defined WINCE
  LARGE_INTEGER freq;
  QueryPerformanceFrequency(&freq);
  return (double)freq.QuadPart;
  