Backport fix for building unit-tests for gtest itself with Mingw

[andoks]

backport of bugfix for google#708 from upstream googletest.

fix #9

[andoks]

https://ci.appveyor.com/project/andoks/googletest

[ruslo]

Release: https://github.com/hunter-packages/googletest/releases/tag/1.8.0-hunter-p3

Testing:

• https://ci.appveyor.com/project/ingenue/hunter/build/1.0.860
• https://travis-ci.org/ingenue/hunter/builds/155816959

[ruslo]

https://ci.appveyor.com/project/andoks/googletest

Well, this test use Visual Studio:

-- Building for: Visual Studio 14 2015
-- The C compiler identification is MSVC 19.0.24213.1
-- The CXX compiler identification is MSVC 19.0.24213.1
-- Check for working C compiler using: Visual Studio 14 2015
-- Check for working C compiler using: Visual Studio 14 2015 -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler using: Visual Studio 14 2015
-- Check for working CXX compiler using: Visual Studio 14 2015 -- works

MinGW still not working:

• https://ci.appveyor.com/project/ingenue/hunter/build/1.0.860/job/2rvaic4wy6penu6w

I've saved this pull request in branch hunter.pr-10 and reverted hunter branch to the previous state.

[andoks]

Sorry, I'm not too familiar with appveyor, and obviously did not take enough time to set up building with MinGW properly.

It seems like even though the upstream PR was accepted, it still has not fixed all the issues with building with MinGW on windows (google#721).

An in addition, I'm still running with self-patched hunter (#8), and did not test the cherry-pick locally.

If I submit another PR, I'll make sure to set up testing properly with Appveyour.

[ruslo]

Here is the configuration with AppVeyor testing with MinGW: https://github.com/forexample/hunter-simple/blob/master/appveyor.yml. By the way you can send me pull request with working configuration (or may be even upstream).