Defining cxx_details::auto_ptr<> as either std::auto_ptr<> or
std::unique_ptr<> once instead of checking for SOCI_HAVE_CXX_C11 simplifies
the code and also allows to use unique_ptr<> for MSVS 2010+ which has it even
when SOCI_HAVE_CXX_C11 is not defined.

Also use cxx_details::auto_ptr<> instead of std::auto_ptr<> in the tests to
fix the build in C++11 mode with clang 3.8 due to -Wdeprecated-declarations
warnings given by it for std::auto_ptr<> (and -Werror used by CMake and which
apparently can't be overridden from command line...).