add macOS implementation

macos/src/TimeIt.cpp

@@ -0,0 +1,30 @@
+#include "TimeIt.hpp"
+
+#if __cplusplus <= 199711L
+
+void Timer::reset()
+{
+  clock_gettime(CLOCK_REALTIME, &beg_);
+}
+
+double Timer::elapsed()
+{
+  clock_gettime(CLOCK_REALTIME, &end_);
+  return end_.tv_sec - beg_.tv_sec +
+      (end_.tv_nsec - beg_.tv_nsec) / 1000000000.;
+}
+
+#else
+
+void Timer::reset()
+{
+  beg_ = clock_::now();
+}
+
+double Timer::elapsed()
+{
+  return std::chrono::duration_cast<second_>
+      (clock_::now() - beg_).count();
+}
+
+#endif