Fix unused-function warning with Eigen < 3.2.2.

- CreateBlockJacobian() is only ever used when Eigen >= 3.2.2 is
  detected, but was previously defined whenever CERES_USE_EIGEN_SPARSE
  was defined with no check on the Eigen version.
- This resulted in an unused-function compile warning that became an
  error due to -Werror, preventing compilation when using Eigen < 3.2.2.

Change-Id: I24628ff329f14b087ece66bf2626bdc0de4ba224

internal/ceres/reorder_program.cc

diff --git a/internal/ceres/reorder_program.cc b/internal/ceres/reorder_program.cc
index 23f301a..aa3d4ce 100644
--- a/internal/ceres/reorder_program.cc
+++ b/internal/ceres/reorder_program.cc
@@ -78,7 +78,7 @@
   return min_parameter_block_position;
 }
 
-#ifdef CERES_USE_EIGEN_SPARSE
+#if EIGEN_VERSION_AT_LEAST(3, 2, 2) && defined(CERES_USE_EIGEN_SPARSE)
 Eigen::SparseMatrix<int> CreateBlockJacobian(
     const TripletSparseMatrix& block_jacobian_transpose) {
   typedef Eigen::SparseMatrix<int> SparseMatrix;