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

[Drew Parsons]

Thinking about it, if possible it would be best to have hdf5-mpi.pc 
managed by update-alternatives, but tracking the mpi alternative.

i.e. set up as a slave alternative depending on mpi, that updates if the 
local system switches mpi from openmpi to mpich.

I'm not sure if this can be done cross-package, but good if it can.

If it can be done, then the control logic would go in the postinst/prerm 
of libhdf5-openmpi-dev and libhdf5-mpich-dev rather than 
libhdf5-mpi-dev.