Bug#953021: libhdf5-mpi-dev: provide default mpi alternatives (hdf5-mpi.pc)

[Drew Parsons]

Source: hdf5
Followup-For: Bug #953021
Control: retitle -1 libhdf5-mpi-dev: provide links to default mpi (hdf5-mpi.pc)

"alternative" here is the sense of which mpi implementation, not in
the sense of "update-alternatives --config hdf5-mpi.pc"

(but separate from this bug, this hdf5-mpi.pc should be registered against
the hdf5.pc alternative, see Bug#953020)