OGS_USE_EIGEN_UNSUPPORTED=OFF does not build
With !4730 (merged) builds based on OGS_USE_EIGEN_UNSUPPORTED=OFF
such as the guix container jobs fail.
Job #363253 failed for 1747ca89:
In file included from /tmp/guix-build-ogs-ssd-6.4.99-git.drv-0/build/MathLib/CMakeFiles/MathLib.dir/Unity/unity_1_cxx.cxx:17:
/tmp/guix-build-ogs-ssd-6.4.99-git.drv-0/source/MathLib/LinAlg/Eigen/EigenLinearSolver.cpp: In member function ‘bool MathLib::EigenLinearSolverBase::solve(MathLib::EigenLinearSolverBase::Vector&, MathLib::EigenLinearSolverBase::Vector&, MathLib::EigenOption&)’:
/tmp/guix-build-ogs-ssd-6.4.99-git.drv-0/source/MathLib/LinAlg/Eigen/EigenLinearSolver.cpp:55:13: error: ‘scaling_’ was not declared in this scope
if (scaling_)