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

Fix the compiler error which prevents deriving from std::iterator in C++17

It was incorrectly left at 0.6.0 when 0.6.1 was released, so this needs to be done to make it consistent between the tagged version and the CMake version.

pkg-config should be giving absolute paths rather than relative paths.

* Change None in C/C++ source to NoneType.

Prevent name conflict with X11 None definition.

Don't build tests if the confusingly named YAML_CPP_BUILD_TOOLS is ON.
Instead, add a new option that controls only if the tests are built.
(Also, default tests to OFF.)

Fix some virtual methods of NullEventHandler that were apparently
intended to overload base class methods, but did not. Add override
keyword to prevent these getting out of sync again.

Add explicit virtual dtor to GraphBuilderInterface. This avoids tripping
a -Wnon-virtual-dtor warning, and also ensures that the correct
(virtual) dtor is called if an instance is deleted via a pointer to the
base type.

Remove use of CMAKE_INSTALL_PREFIX when specifying install destinations.
It is preferred that install destinations are specified as relative
paths, not absolute paths. This is also needed to create relocatable
installs, and thus should fix #526.

Don't install LIBRARIES on Windows to `bin`; they belong in `lib`. (This
appears to have been done in a mistaken attempt to install the DLL's to
`bin`, which do need to be there, but RUNTIME DESTINATION already takes
care of this.)

The combination of clang++ 3.4 with libstdc++ from GCC 4.6
doesn't work.

Note that with the release of 1.8.0, googlemock and
googletest are unified into a single release.

This way it's possible to build using older MSVC (<13) that don't
support this yet. Macro is undefined in each file where it is used so it
should stack well with other libs and sources.

Taken from https://github.com/google/googletest/issues/705#issuecomment-235067917

* fix two compile warnings in public headers

both warnings have to do with variable
name shadowing

* Fixed compiler warning -Wdeprecated with clang.

Starting with C++11 implicit copy-constructors are deprecated when the class
has a user defined destructor.

* Fixes -Wdocumentation warning.

yaml-cpp/parser.h:50:65: warning: parameter 'eventHandler}.' not found in the function
      declaration [-Wdocumentation]
   * Handles the next document by calling events on the {@param eventHandler}.
                                                                ^~~~~~~~~~~~~~
yaml-cpp/parser.h:50:65: note: did you mean 'eventHandler'?
   * Handles the next document by calling events on the {@param eventHandler}.
                                                                ^~~~~~~~~~~~~~
                                                                eventHandler

Previously, just referencing the next element in the sequence (and so constructing it, as an undefined element) would allow you to skip defining an element without turning the sequence into a map. E.g:

node[0] = "foo"; // sequence of size 1
node[1]; // sequence of size 1, with an undefined element at 1
node[2] = "bar"; // FIX: should be map of size 2 (since there's no element at index 1)