math/gmm++: unbreak build of consumer (cad/gmsh)

We need to define which linear equations solver (MUMPS or SuperLU) to
be used in configuration header. This commit adds either of them based
on a chosen option and should unbreak the build of cad/gmsh.

PR: 288106
Reported by: thierry
Approved by: Atanu Biswas <atanubiswas484@gmail.com> (maintainer)
Fixes: 5b6b3839ef38 (update to 5.4.4 and take maintainership)