Fix Pybind11Extension on mingw64 (#2921)

* Pybind11Extension add the "/EHsc /bigobj /std:c++14" flags on Windows.
  This is good for Visual C++ but not for Mingw.
* According
  https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-python2/0410-MINGW-build-extensions-with-GCC.patch
  sysconfig.get_platform() is the way to check for a Mingw64
1 file changed