Bug#989497: superlu-dist: provide 64-bit build

[Drew Parsons]

Source: superlu-dist
Severity: normal
Control: block -1 by 961183
Control: block 961977 by -1

We've started providing 64-bit builds of the numerical computational
libraries, in order to support computations of extreme systems with
more than a billion degrees of freedom.

64-bit PETSc is already in Debian.  It built against standard
superlu-dist, but that is actually a configuration error. 64-bit PETSc
requires 64-bit SuperLU-Dist.

The 64-bit build of SuperLU-Dist is easily activated with
-DXSDK_INDEX_SIZE=64. But it will require 64-bit metis.