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This reverts commit 3e33bb31.

The original commit broke the build (#612) when yaml-cpp is used as a git submodule.

* Move enable_testing() into proper place

* Added CMake's checks for C++11 standards

Raised minimal version of CMake to 3.1, since on old systems there no
decent compilers that supports c++11.

Closes #377.

* Externalize googletest project

Externalize gtest to avoid installation, fixes #539.

* Remove defined cmake_policies

CMP0012 - OLD marked as deprecated for >=cmake-3.1 and will be removed
CMP0015 - does not affect to build process
CMP0042 - already NEW for >=cmake-3.1

Fixes #505

* Fix compiling in Windows MSVC

 - Adds 'std=c++11' compiler flags
 - Replaces boost::type_traits with std::type_traits
 - Replaces boost::shared_ptr with std::shared_ptr
 - Replaces std::auto_ptr with std::unique_ptr
 - Replaces raw pointers with std::unique_ptr in ptr_vector, ptr_stack, and SettingChanges
 - Replaces boost::noncopyable with deleted copy and assignment operators
 - Replaces boost::next with std::next
 - Replaces boost::enable_if with std::enable_if
 - Replaces boost::is_convertible with std::is_convertible
 - Replaces ptrdiff_t with std::ptrdiff_t
 - Replaces boost::iterator_facade and boost::iterator_adaptor with std::iterator, borrowing the 'proxy reference' technique from boost
 - Removes Boost dependency from CMakeLists
 - Formats changed files using clang-format