Ensure PYTHONQT_INSTALL_DIR is not reset in project including CTK
Ensure PYTHONQT_INSTALL_DIR is not reset in project including CTK

This commit is an attempt to fix a regression introduced by fc92239
that was causing the Slicer packaging to fail (mainly because the
variable PYTHONQT_INSTALL_DIR was reset to an empty value after
"find_path" was called in FindPythonQt.cmake)

Note that this a workaround, the issue is being discussed with CMake
developer.

naucoin/CTK,ddao/CTK,AndreasFetzer/CTK,danielknorr/CTK,CJGoch/CTK,CJGoch/CTK,naucoin/CTK,rkhlebnikov/CTK,Sardge/CTK,Heather/CTK,sankhesh/CTK,fedorov/CTK,jcfr/CTK,Sardge/CTK,AndreasFetzer/CTK,151706061/CTK,mehrtash/CTK,danielknorr/CTK,rkhlebnikov/CTK,CJGoch/CTK,Sardge/CTK,ddao/CTK,AndreasFetzer/CTK,ddao/CTK,mehrtash/CTK,fedorov/CTK,sankhesh/CTK,151706061/CTK,jcfr/CTK,mehrtash/CTK,Sardge/CTK,sankhesh/CTK,danielknorr/CTK,rkhlebnikov/CTK,SINTEFMedtek/CTK,rkhlebnikov/CTK,commontk/CTK,CJGoch/CTK,jcfr/CTK,msmolens/CTK,msmolens/CTK,naucoin/CTK,fedorov/CTK,jcfr/CTK,151706061/CTK,Heather/CTK,danielknorr/CTK,commontk/CTK,sankhesh/CTK,151706061/CTK,SINTEFMedtek/CTK,151706061/CTK,laurennlam/CTK,AndreasFetzer/CTK,msmolens/CTK,SINTEFMedtek/CTK,CJGoch/CTK,mehrtash/CTK,SINTEFMedtek/CTK,msmolens/CTK,naucoin/CTK,commontk/CTK,Heather/CTK,jcfr/CTK,laurennlam/CTK,laurennlam/CTK,fedorov/CTK,laurennlam/CTK,SINTEFMedtek/CTK,ddao/CTK,commontk/CTK,Heather/CTK