Patches for sip / PyQt-builder for mingw cross-compilation of python bindings

[Sandro Mani]

> Can you explain why this is needed?
>
>> @@ -467,8 +469,9 @@ macx {
>>          # TODO: is this still necessary for Python v3.8?
>>          if not buildable.static:
>>              pro_lines.extend(['win32 {',
>> -                    '    LIBS += -L{}'.format(
>> - self.qmake_quote(project.py_pylib_dir)),
>> +                    '    LIBS += -L{libdir} -l{lib}'.format(
>> + libdir=self.qmake_quote(project.py_pylib_dir),
>> + lib=self.qmake_quote(project.py_pylib_lib)),
>>                      '}'])
>>
>>          # Add any installables from the buildable.
>
I got undefined reference to various symbols defined in libpython. I 
couldn't figure out how else -lpythonX.Y was supposed to be added to the 
link flags, so I added it explicitly.

Thanks
Sandro