replace MPI_Wtime() with platform::walltime()

src/STUBS/mpi.cpp

@@ -21,7 +21,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/time.h>
 
 /* data structure for double/int */
 
@@ -164,23 +163,10 @@ int MPI_Finalize()
 }
 
 /* ---------------------------------------------------------------------- */
-
+static auto initial_time = std::chrono::steady_clock::now();
 double MPI_Wtime()
 {
-#if defined(_MSC_VER)
-  double t;
-
-  t = GetTickCount();
-  t /= 1000.0;
-  return t;
-#else
-  double time;
-  struct timeval tv;
-
-  gettimeofday(&tv, NULL);
-  time = 1.0 * tv.tv_sec + 1.0e-6 * tv.tv_usec;
-  return time;
-#endif
+  return std::chrono::duration<double>(std::chrono::steady_clock::now() - initial_time).count();
 }
 
 /* ---------------------------------------------------------------------- */