Support combination of features `mpio` and `static`

[magnusuMET]

This issue is a placeholder for discussion of how to implement this combination.

Problem in short:
The build in hdf5-src requires an MPI-compatible c-compiler

[aldanor]

A good start might be hdf5 feedstock on conda-forge where they build it with both openmpi and mpich: https://github.com/conda-forge/hdf5-feedstock/tree/master/recipe

[aldanor]

@magnusuMET It looks like mpi-sys needs mpi compiler anyway to do its bindgen-style magic to build itself, so it's not like we're requiring anything extra I think (https://github.com/rsmpi/rsmpi).