Removed distutils from MinGW detection

[radarhere]

Helps #4796

In setup.py, there is

PLATFORM_MINGW = "mingw" in ccompiler.get_default_compiler()

Looking at ccompiler.get_default_compiler() on MinGW -

def get_default_compiler(osname=None, platform=None):
    ...
    if get_platform().startswith('mingw'):
        return 'mingw32'

def get_platform():
    if os.name == 'nt':
        TARGET_TO_PLAT = {
            'x86' : 'win32',
            'x64' : 'win-amd64',
            'arm' : 'win-arm32',
        }
        return TARGET_TO_PLAT.get(os.environ.get('VSCMD_ARG_TGT_ARCH')) or get_host_platform()
    else:
        return get_host_platform()

def get_host_platform():
    ...
    if os.name == 'nt':
        if 'GCC' in sys.version:
            return 'mingw'

Note that I find the code on MinGW slightly different to the one on my machine - mine does not include the MinGW lines.

So then the setup.py line can be replaced with

PLATFORM_MINGW = os.name == "nt" and "GCC" in sys.version

[hugovk]

Thanks!