Fix Cygwin CMake Build

[MaEtUgR]

Why?
Based on the findings I wrote here I'm fixing the global CMakeLists file such that building under Cygwin also works out of the box e.g. when following the readme and including the repo in your CMake project.

On Cygwin specifying set(CMAKE_CXX_EXTENSIONS OFF) breaks the build exactly like described in https://stackoverflow.com/q/18784112/6326048

• add_definitions(-std=c++11) also breaks it
• Removing the whole block of compile std defines builds fine

@peterjc123 Is set(CMAKE_CXX_EXTENSIONS OFF) really necessary for you? If yes I'll create a pr to only exclude the line for Cygwin to makes sure it builds out of the box on that platform as well.

What?

• First commit To detect Cygwin I had to move the project initialization to the beginning, see https://stackoverflow.com/a/35914521/6326048
• Second commit I'm not disabling C++ extensions when building the repository using the global CMake file in a Cygwin environment.

Really?
Here's the output after the following commands in the freshly cloned repo wit the latest Cygwin environment:

mkdir build
cd build
cmake ..
make

Before change:

After change:

Additional info
Building gtest without any C++ standard definitions works fine on Cygwin, probably the defaults are just right. That's also why diectly building it using the CMakeLists.txt files in the googletest or googlemock folder is fine. I'm fixing the global one because I intend to use it and it's also frustrating to new users to first have a failing build.

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[MaEtUgR]

I signed it! 😃

[googlebot]

CLAs look good, thanks!

[gennadiycivil]

merged now

[MaEtUgR]

@gennadiycivil I just tested and it works perfect thanks! I could probably contribute CI on Cygwin using appveyor if there's any interest.