do not define fallback for MPI_Comm since this might cause side effects when linking against HDF5

src/sct_mach.h

@@ -103,13 +103,6 @@ static int inline omp_get_max_threads() { return 1; }
 // MPI:
 #ifdef HAVE_MPI
 #  include <mpi.h>
-#else
-/*! \brief fall back definitions if we don't have MPI:
-           use int for communicator */
-typedef int MPI_Comm;
-enum { MPI_COMM_WORLD };
-enum { MPI_COMM_SELF };
-enum { MPI_DOUBLE };
 #endif
 //public-