Require C++11 to build CMake itself

CMake can now compile as C++11 on all supported platforms.  Check that
std::unique_ptr is available and fail early if missing.  This will allow
us to use C++11 more broadly in CMake's implementation (previously it
was restricted to the serve mode implementation).

Co-Author: Daniel Pfeifer <daniel@pfeifer-mail.de>
5 files changed