Import also MPI C component in cmake config

Henning Glawerequested to merge
cmake_import_mpi_component_C into hotfix-16.1

Description

In cmake builds, load also MPI C component and not only the Fortran one.

scalapack 2.2.2 cmake config references MPI::MPI_C in target_link_libraries, without calling find_package( MPI ) by itself. So it expects the consuming package to do this.

No negative impact expected (C is the default interface anyways).

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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