added check for MPI

configure.ac

@@ -236,6 +236,12 @@ fi
 
 AC_MSG_NOTICE([finding libraries])
 
+# Ensure we have MPI.
+AC_CHECK_FUNCS([MPI_Init])
+if test "x$ac_cv_func_MPI_Init" != "xyes"; then
+   AC_MSG_ERROR([Can't link to MPI library. MPI is required.])
+fi
+
 # Check for netCDF library.
 AC_CHECK_LIB([netcdf], [nc_create], [], [AC_MSG_ERROR([Can't find or link to the netcdf library.])])
 AC_CHECK_HEADERS([netcdf.h netcdf_meta.h])
@@ -333,9 +339,6 @@ else
    AC_MSG_ERROR([Unable to find type MPI_Offset in mpi.h])
 fi
 
-#AC_CHECK_SIZEOF([MPI_Offset], [], [[#include <mpi.h>]])
-#AC_DEFINE([SIZEOF_MPI_OFFSET], [8], [netCDF classic library available])
-
 # If we want the timing library, we must find it.
 if test "x$enable_timing" = xyes; then
    AC_CHECK_HEADERS([gptl.h])