Change the defaults for shared_ptr.

By default shared_ptr is now assumed to be
in the standard <memory> header and in the
std namespace.

Previously the way the ifdefs were structured if the appropriate
variable was not defined, it would default to <t1/memory>.

The new defaults are more future proof.

Change-Id: If457806191196be2b6425b8289ea7a3488a27445

include/ceres/internal/port.h

diff --git a/include/ceres/internal/port.h b/include/ceres/internal/port.h
index 721996c..4d0bbc0 100644
--- a/include/ceres/internal/port.h
+++ b/include/ceres/internal/port.h
@@ -33,13 +33,12 @@
 
 #include <string>
 
-#if defined(CERES_STD_SHARED_PTR) || defined(CERES_STD_SHARED_PTR_IN_TR1_NAMESPACE)
-#include <memory>
-#else
+#if defined(CERES_TR1_SHARED_PTR)
 #include <tr1/memory>
+#else
+#include <memory>
 #endif
 
-
 namespace ceres {
 
 // It is unfortunate that this import of the entire standard namespace is
@@ -52,12 +51,10 @@
 // "string" implementation in the global namespace.
 using std::string;
 
-#if defined(CERES_STD_SHARED_PTR)
-using std::shared_ptr;
-#endif
-
 #if defined(CERES_STD_SHARED_PTR_IN_TR1_NAMESPACE) || defined(SHARED_PTR_IN_TR1_NAMESPACE)
 using std::tr1::shared_ptr;
+#else
+using std::shared_ptr;
 #endif
 
 }  // namespace ceres