gnu: mumps: Build and install shared libraries.

gnu: mumps: Build and install shared libraries.

* gnu/packages/patches/mumps-shared-libseq.patch,
gnu/packages/patches/mumps-shared-mumps.patch,
gnu/packages/patches/mumps-shared-pord.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.
* gnu/packages/maths.scm (mumps)[source](patches): Add them.
[arguments]: In 'configure' phase, adjust "Makefile.inc" for shared
library support.  In 'install' phase, install libmpiseq.so when it
exists.

Co-authored-by: Ludovic Courtès <ludovic.courtes@inria.fr>