[PyQt] Stub files install location issues with configure.py

[Alvin Wong]

Hi,

I recently discovered that the stub files gets installed to the
default Python site-packages directory even if the `--destdir` option
has been passed to configure.py. From the page
http://pyqt.sourceforge.net/Docs/PyQt5/installation.html#building-and-installing-from-source,
it is written this way:

> --stubdir <DIR>
>
> The PEP 484 type hint stub files for the PyQt5 modules will be installed in the directory <DIR>. By default they will be stored in the same directory as the corresponding extension modules.

The explanation seems to imply that the stub files will get installed
to `<destdir>/PyQt5`, but they aren't. It makes me think either there
is a bug with the configure script or the documentation needs to be
clarified.

Another issue I found is that, the actual option the configure script
uses is `--stubsdir` with an 's'. Please also fix this in the
documentation.

Best Regards,
Alvin