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/schur_eliminator_test.cc b/internal/ceres/schur_eliminator_test.cc
index 0ac4354..5146d47 100644
--- a/internal/ceres/schur_eliminator_test.cc
+++ b/internal/ceres/schur_eliminator_test.cc
@@ -32,6 +32,7 @@
 
 #include <glog/logging.h>
 #include "ceres/file.h"
+#include "gflags/gflags.h"
 #include "gtest/gtest.h"
 #include "Eigen/Dense"
 #include "ceres/block_random_access_dense_matrix.h"