Fix build breakage when LAPACK support is disabled.

Parts of dense_cholesky.h/cc need to be enabled/disabled using
CERES_NO_LAPACK.

Fixes https://github.com/ceres-solver/ceres-solver/issues/744

Change-Id: Id555e16723e31580eaa81562dba8d745a9210b0a

internal/ceres/dense_cholesky.cc

diff --git a/internal/ceres/dense_cholesky.cc b/internal/ceres/dense_cholesky.cc
index 1037ad3..148c38c 100644
--- a/internal/ceres/dense_cholesky.cc
+++ b/internal/ceres/dense_cholesky.cc
@@ -118,6 +118,7 @@
   return LINEAR_SOLVER_SUCCESS;
 }
 
+#ifndef CERES_NO_LAPACK
 LinearSolverTerminationType LAPACKDenseCholesky::Factorize(
     int num_cols, double* lhs, std::string* message) {
   lhs_ = lhs;
@@ -171,5 +172,7 @@
   return termination_type_;
 }
 
+#endif  // CERES_NO_LAPACK
+
 }  // namespace internal
 }  // namespace ceres

internal/ceres/dense_cholesky.h

diff --git a/internal/ceres/dense_cholesky.h b/internal/ceres/dense_cholesky.h
index 0859c68..268862d 100644
--- a/internal/ceres/dense_cholesky.h
+++ b/internal/ceres/dense_cholesky.h
@@ -110,6 +110,7 @@
   std::unique_ptr<LLTType> llt_;
 };
 
+#ifndef CERES_NO_LAPACK
 class CERES_EXPORT_INTERNAL LAPACKDenseCholesky : public DenseCholesky {
  public:
   ~LAPACKDenseCholesky() override = default;
@@ -126,6 +127,7 @@
   int num_cols_ = -1;
   LinearSolverTerminationType termination_type_ = LINEAR_SOLVER_FATAL_ERROR;
 };
+#endif  // CERES_NO_LAPACK
 
 }  // namespace internal
 }  // namespace ceres