[cmake] Discover PyQt4 built with its new build system

Addresses bug reports [#10596](https://hub.qgis.org/issues/10596), [#8857](https://hub.qgis.org/issues/8857). The module pyqtconfig isn't available in PyQt4 built with configure-ng.py instead of configure.py, so changes we made in cmake/FindPyQt.py to support both.
Now can build without issues on Arch Linux x86_64.
Related links:
* [PyQt build system](http://pyqt.sourceforge.net/Docs/PyQt4/build_system.html)
* [SIP build system](http://pyqt.sourceforge.net/Docs/sip4/build_system.html)