CMakeLists.txt: provide C++ standart option to compiler using special cmake variables
CMakeLists.txt: provide C++ standart option to compiler using special cmake variables

To fix compilation when integrated CMakeLists.txt also defines required C++ standart.

E.g. fresh versions of GoogleTest requires C++11 in its own CMakeLists.txt.

GSGroup/stingraykit,GSGroup/stingraykit