message: Don't build SLEPc when not asked
diff: #include "../config/config.hpp"
  +#ifdef MFEM_USE_MPI
  +#ifdef MFEM_USE_PETSC
  +#ifdef MFEM_USE_SLEPC
  +
  #include "linalg.hpp"
  #include "slepc.h"
  @@ -332,3 +336,7 @@ void SlepcEigenSolver::SetSpectralTransformation(
  }
  }
  +
  +#endif // MFEM_USE_SLEPC
  +#endif // MFEM_USE_PETSC
  +#endif // MFEM_USE_MPI
  