Remove CMake test for MPI_SEEK_SET or make it more portable

[sloede]

The FindMPI module for CMake includes a test for the presence of MPI_SEEK_SET:

dealii/cmake/modules/FindDEAL_II_MPI.cmake

Lines 65 to 74 in e8e4301

	CHECK_CXX_SOURCE_COMPILES(
	"
	#include <mpi.h>
	#ifndef MPI_SEEK_SET
	# error
	#endif
	int main() {}
	"
	MPI_HAVE_MPI_SEEK_SET
	)

The test requires MPI_SEEK_SET to be a preprocessor definition, which is more restrictive than what the MPI standard mandates. This causes problems with MPI implementations such as MPItrampoline, which defines it as

extern int MPI_SEEK_SET;

A quick grep on the sources for deal.II v9.4.1 shows that MPI_SEEK_SET does not seem to be used (and neither are MPI_SEEK_CUR or MPI_SEEK_END). Thus my question is: Would it be possible to remove this test altogether? Or, if it is still needed, could it be changed to something that would work for MPItrampoline, e.g.,

CHECK_CXX_SOURCE_COMPILES(
  "
  #include <mpi.h>
  int main() {
    MPI_SEEK_SET;
  }
  "
  MPI_HAVE_MPI_SEEK_SET
  )

cc @eschnett

[masterleinad]

The last occurrence was removed in abbca77. I would think it's OK to remove the check now.