use python with mingw on windows
The [closed issues](https://github.com/Pebaz/nimporter/issues/37) made [a change](<https://github.com/Pebaz/nimporter/commit/adbd2404473ef12968faa5e51f8f54424e69103c>) as
```python
    - (['--cc:vcc'] if sys.platform == 'win32' else [])
    + (['--cc:vcc'] if shutil.which('vccexe') else [])
```
which looks reasonable, but should it be( sorry I have not use MSVC for a long time, cl.exe is valid for VC 6.0)
```python
    - (['--cc:vcc'] if sys.platform == 'win32' else [])
    + (['--cc:vcc'] if shutil.which('cl.exe') else [])
```

however in current code, this change is lost again:
```python
    NIM_CLI_ARGS = [
                       '--opt:speed',
                       '--parallelBuild:0',
                       '--gc:refc',
                       '--threads:on',
                       '--app:lib',
                       '-d:release',
                       '-d:strip',
                       '-d:ssl',

                       # https://github.com/Pebaz/nimporter/issues/41
                       '--warning[ProveInit]:off',

                   ] + (['--cc:vcc'] if 'MSC' in sys.version else [])
```
which is definitely wrong because sys.version tells which C compiler is used to compile the `python.exe`, but can not tell whether `python.exe` runs in MSVC or mingw circumstance