This fix selects std::unique_ptr when the compiler is identified as being C++11
 compliant.

To ensure the compiler is C++11 compliant or better, __cplusplus, _MSC_VER and
_CPPLIB_VER are tested.
_MSC_VER is used since Microsoft compilers still use 19711L
 as the __cplusplus value.
_CPPLIB_VER is used to test for better Dinkumware support.

The variable VTK_HAS_STD_UNIQUE_PTR is defined if std::unique_ptr is available.