Bug#972069: slepc64-dev: enable ARPACK support in 64-bit build

[Drew Parsons]

Source: slepc
Followup-For: Bug #972069

I don't think we'll be able to resolve this request.

64-bit arpack is now available in libarpack64-2-dev

But slepc's arpack configuration uses parpack when MPI support is active


64-bit support in ARPACK is explicitly excluded from PARPACK,
see
https://github.com/opencollab/arpack-ng/blob/804fa3149a0f773064198a8e883bd021832157ca/configure.ac#L196

i.e. libparpack64.so is not available
and we therefore cannot activate 64-bit ARPACK for SLEPc.