python-cython C++ support patch

[Masamichi Hosoda]

>> python38-cython-0.29.21-3 works fine with following test code.
>> 
>> ```hello.pyx
>> print('Hello World!')
>> ```
>> 
>> ```setup.py
>> from distutils.core import setup
>> from Cython.Build import cythonize
>> 
>> setup(
>>      ext_modules=cythonize(
>>          "hello.pyx",
>>          language="c++",
>>      )
>> )
>> ```
>> 
>> ```test.py
>> import hello
>> ```
>> 
>> Here's the result by python38-cython-0.29.21-2 (failed).
>> 
>> ```
>> $ python setup.py build_ext --inplace
>> [...snip...]
>> $ python test.py
>> Traceback (most recent call last):
>>    File "test.py", line 1, in <module>
>>      import hello
>> ImportError: dynamic module does not define module export function (PyInit_hello)
>> ```
>> 
>> Here's the result by python38-cython-0.29.21-3 (succeeded).
>> 
>> ```
>> $ python setup.py build_ext --inplace
>> [...snip...]
>> $ python test.py
>> Hello World!
>> ```
>> 
>> Thanks.
>
> Thanks to you
>
> I still think that the change you proposed should go upstream.
>
> Have you tested your example on a NOT cygwin system ?
>
> Regards
> Marco

Here's the result by Ubuntu 20.04 with python-cython 0.29.21

```
$ lsb_release -d
Description:    Ubuntu 20.04.2 LTS
$ pip3 list | grep "Cython"
Cython                 0.29.21
$ python3 setup.py build_ext --inplace
[...snip...]
$ python3 test.py
Hello World!
```

The export name is different for the successful and unsuccessful cases.

Here's Cygwin python38-cython-0.29.21-2 (failed).

```
$ nm hello.cpython-38-x86_64-cygwin.dll | grep "PyInit"
000000040b8a22af T _Z12PyInit_hellov
```

Here's Cygwin python38-cython-0.29.21-3 (succeeded).

```
$ nm hello.cpython-38-x86_64-cygwin.dll | grep "PyInit"
000000040b8a22af T PyInit_hello
```

Here's Ubuntu python-cython 0.29.21 (succeeded).

```
$ nm hello.cpython-38-x86_64-linux-gnu.so | grep "PyInit"
00000000000030b7 T PyInit_hello
```

Thanks.