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/linear_least_squares_problems.cc b/internal/ceres/linear_least_squares_problems.cc
index fccc3b1..ef72cbd 100644
--- a/internal/ceres/linear_least_squares_problems.cc
+++ b/internal/ceres/linear_least_squares_problems.cc
@@ -61,6 +61,7 @@
     default:
       LOG(FATAL) << "Unknown problem id requested " << id;
   }
+  return NULL;
 }
 
 #ifndef CERES_DONT_HAVE_PROTOCOL_BUFFERS