Bug#1082085: libsundials-dev: sundials_config.h sets SUNDIALS_MPI_ENABLED=1 even for serial jobs

[Drew Parsons]

The issue is known upstream, and in fact is deliberate.  In that sense it is not a bug, but may be
an issue for reverse dependencies.

The change in behaviour comes from the introduction of SUNComm in PR#370
https://github.com/LLNL/sundials/pull/370
committed at https://github.com/LLNL/sundials/commit/947ad4b68dc8add194422237bc16934324ac0730

>From the commit description:
"Requires MPI to be linked to SUNDIALS core if it is enabled thus all-in-one MPI and non-MPI builds are no longer supported. Instead user will have to separately build and install with and without MPI."

To keep both would require packaging a separate serial build, as is done with HDF5 and ADIOS2.
This will require a bit of Debian maintainer work, which I personally an not in a rush to do.

Upstream mainly expects client apps to use cmake, in which case there is no operational problem (just means such applications
will be linked to libmpi.so even if they're not using MPI directly).

Some applications such as opendrop do not use cmake. If we keep sundials 7 as-is (with MPI support), then these
non-cmake apps will need to be patched to enable access to mpi.h. Setting CXX=mpicxx might be sufficient.

It'probably s reasonable for the default build oSUNDIALS to support MPI.
More complex client applications such as deal.ii would want it.