Conditionally disable SparseNormalCholesky.

Disable creation of SparseNormalCholesky if CXSparse and
SuiteSparse are not evailable.

Change-Id: I9a3beafef0073d1c55305beb0af31bc67ef39691

internal/ceres/linear_solver.cc

diff --git a/internal/ceres/linear_solver.cc b/internal/ceres/linear_solver.cc
index 7339205..e983e2c 100644
--- a/internal/ceres/linear_solver.cc
+++ b/internal/ceres/linear_solver.cc
@@ -75,10 +75,18 @@
       return new CgnrSolver(options);
 
     case SPARSE_NORMAL_CHOLESKY:
+#if defined(CERES_NO_SUITESPARSE) && defined(CERES_NO_CXSPARSE)
+      return NULL;
+#else
       return new SparseNormalCholeskySolver(options);
+#endif
 
     case SPARSE_SCHUR:
+#if defined(CERES_NO_SUITESPARSE) && defined(CERES_NO_CXSPARSE)
+      return NULL;
+#else
       return new SparseSchurComplementSolver(options);
+#endif
 
     case DENSE_SCHUR:
       return new DenseSchurComplementSolver(options);