Bug#980751: petsc: breaks multiple autopkgtests: undefined reference to 'MPIU_SUM'

[Graham Inggs]

Control: severity -1 important

Indeed, these autopkgtests were fixed by the binNMU of slepc.

Lowering severity so openmpi and petsc can migrate, but we need to
figure out what happened here so we can prevent it in future.

Does slepc need a tighter dependency on the version of slepc that it
was built against?