FTE compilation in our gitian setup is broken for Windows with GCC 6.2.0

After #13893 (moved) landed (bump mingw-w64 and GCC to 6.2.0) it turns out that FTE compilation is broken:

+ make
wine /home/ubuntu/install/python/python.exe setup.py build_ext -c mingw32
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
running build_ext
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
fixme:msvcrt:MSVCRT__wsopen_s : pmode 0x81b6 ignored
building 'fte.cDFA' extension
creating build
creating build\temp.win32-2.7
creating build\temp.win32-2.7\Release
creating build\temp.win32-2.7\Release\fte
C:\windows\gcc.exe -mno-cygwin -mdll -O -Wall -Ifte -Ithirdparty/gmp/include -IZ:\home\ubuntu\install\python\include -IZ:\home\ubuntu\install\python\PC -c fte/rank_unrank.cc -o build\temp.win32-2.7\Release\fte\rank_unrank.o -O3 -fPIC
Exception WindowsError: (6, 'Invalid handle') in <bound method Popen.__del__ of <subprocess.Popen object at 0x00159130>> ignored
C:\windows\gcc.exe -mno-cygwin -mdll -O -Wall -Ifte -Ithirdparty/gmp/include -IZ:\home\ubuntu\install\python\include -IZ:\home\ubuntu\install\python\PC -c fte/cDFA.cc -o build\temp.win32-2.7\Release\fte\cdfa.o -O3 -fPIC
In file included from /home/ubuntu/install/mingw-w64/i686-w64-mingw32/include/c++/6.2.0/math.h:36:0,
                 from /home/ubuntu/.wine/dosdevices/z:/home/ubuntu/install/python/include/pyport.h:325,
                 from /home/ubuntu/.wine/dosdevices/z:/home/ubuntu/install/python/include/Python.h:58,
                 from /home/ubuntu/.wine/dosdevices/z:/home/ubuntu/build/libfte/fte/cDFA.cc:1:
/home/ubuntu/install/mingw-w64/i686-w64-mingw32/include/c++/6.2.0/cmath:1133:11: error: '::hypot' has not been declared
   using ::hypot;
           ^~~~~
In file included from /home/ubuntu/.wine/dosdevices/z:/home/ubuntu/install/python/include/Python.h:80:0,
                 from /home/ubuntu/.wine/dosdevices/z:/home/ubuntu/build/libfte/fte/cDFA.cc:1:
/home/ubuntu/.wine/dosdevices/z:/home/ubuntu/build/libfte/fte/cDFA.cc: In function 'void initcDFA()':
/home/ubuntu/.wine/dosdevices/z:/home/ubuntu/install/python/include/object.h:767:22: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
     ((PyObject*)(op))->ob_refcnt++)

                      ^
/home/ubuntu/.wine/dosdevices/z:/home/ubuntu/build/libfte/fte/cDFA.cc:273:5: note: in expansion of macro 'Py_INCREF'
     Py_INCREF(&DFAType);
     ^~~~~~~~~
Exception WindowsError: (6, 'Invalid handle') in <bound method Popen.__del__ of <subprocess.Popen object at 0x00159150>> ignored
error: command 'gcc' failed with exit status 1
fixme:msvcrt:__clean_type_info_names_internal (0x1d114810) stub
fixme:msvcrt:__clean_type_info_names_internal (0xa254f0) stub
fixme:msvcrt:__clean_type_info_names_internal (0x100d4460) stub
fixme:msvcrt:__clean_type_info_names_internal (0x42ba30) stub
fixme:msvcrt:__clean_type_info_names_internal (0x1e24c178) stub
make: *** [fte/cDFA.pyd] Error 1