Decl'HDF5 tests fail when building in serial

/tmp/src/pdiplugin-decl-hdf5-9999master/plugins/decl_hdf5/tests/compatibility_tests/HDF5_C/mpi_write_test.c:27:10: fatal error: mpi.h: No such file or directory
   27 | #include <mpi.h>
      |          ^~~~~~~
compilation terminated.

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