Port Ceres to Windows

This is a preliminary, but full, port of Ceres to Windows.
Currently all tests compile and run, with only system_test
failing to work correctly due to a path issue.

Change-Id: I4152c1588bf51ffd7f4d9401ef9759f5d28c299c
diff --git a/internal/ceres/sparse_normal_cholesky_solver.cc b/internal/ceres/sparse_normal_cholesky_solver.cc
index 448bbd5..a7c43ef 100644
--- a/internal/ceres/sparse_normal_cholesky_solver.cc
+++ b/internal/ceres/sparse_normal_cholesky_solver.cc
@@ -150,6 +150,9 @@
     const LinearSolver::PerSolveOptions& per_solve_options,
     double * x) {
   LOG(FATAL) << "No CXSparse support in Ceres.";
+
+  // Unreachable but MSVC does not know this.
+  return LinearSolver::Summary();
 }
 #endif
 
@@ -233,6 +236,9 @@
     const LinearSolver::PerSolveOptions& per_solve_options,
     double * x) {
   LOG(FATAL) << "No SuiteSparse support in Ceres.";
+
+  // Unreachable but MSVC does not know this.
+  return LinearSolver::Summary();
 }
 #endif