Fix sip dir on arch linux (/usr/share/sip/PyQt4/)

This PR fix a configuration issue when installing **pygs** on **ArchLinux**. The issue is that the sip files are not located under the PyQt directory but under ``/usr/share/sip/PyQt4/``. The fix is to look for QtCoremode.sip under the PyQt directory and to adapt the ``pyqt_sip_dir`` to ``/usr/share/sip/PyQt4`` if the sip file could not be found.

I've also added a way to specify the target qt_api. The issue is that if you have both PyQt5 and PyQt4 installed on your system, pygs/configure.py will automatically pick up PyQt5. Now it will read the ``QT_API`` environment variable (which is set to ``pyqt5`` by default) to import the correct API.