[Math/Eigen] better error message when PardisoLU is not available

MathLib/LinAlg/Eigen/EigenLinearSolver.cpp

@@ -178,13 +178,17 @@ EigenLinearSolver::EigenLinearSolver(
             _solver = details::createIterativeSolver(_option.solver_type,
                                                      _option.precon_type);
             return;
-#ifdef USE_MKL
         case EigenOption::SolverType::PardisoLU: {
+#ifdef USE_MKL
             using SolverType = Eigen::PardisoLU<EigenMatrix::RawMatrixType>;
             _solver.reset(new details::EigenDirectLinearSolver<SolverType>);
             return;
-        }
+#else
+            OGS_FATAL(
+                "The code is not compiled with Intel MKL. Linear solver type "
+                "PardisoLU is not available.");
 #endif
+        }
     }
 
     OGS_FATAL("Invalid Eigen linear solver type. Aborting.");

MathLib/LinAlg/Eigen/EigenOption.cpp

@@ -29,10 +29,8 @@ EigenOption::SolverType EigenOption::getSolverType(const std::string &solver_nam
         return SolverType::BiCGSTAB;
     if (solver_name == "SparseLU")
         return SolverType::SparseLU;
-#ifdef USE_MKL
     if (solver_name == "PardisoLU")
         return SolverType::PardisoLU;
-#endif
 
     OGS_FATAL("Unknown Eigen solver type `%s'", solver_name.c_str());
 }

MathLib/LinAlg/Eigen/EigenOption.h

@@ -23,10 +23,8 @@ struct EigenOption final
     {
         CG,
         BiCGSTAB,
-        SparseLU
-#ifdef USE_MKL
-        , PardisoLU
-#endif
+        SparseLU,
+        PardisoLU
     };
 
     /// Preconditioner type