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

pybind11/setup_helpers.py

@@ -47,6 +47,7 @@
 import threading
 import platform
 import warnings
+import sysconfig
 
 try:
     from setuptools.command.build_ext import build_ext as _build_ext
@@ -59,7 +60,7 @@
 import distutils.ccompiler
 
 
-WIN = sys.platform.startswith("win32")
+WIN = sys.platform.startswith("win32") and sysconfig.get_platform() != "mingw"
 PY2 = sys.version_info[0] < 3
 MACOS = sys.platform.startswith("darwin")
 STD_TMPL = "/std:c++{}" if WIN else "-std=c++{}"