STYLE: Remove IGT references from Libs/CMakeLists.txt

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21726 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Add experimental support for Qt 4.8.4

The build system now checks with the discovered Qt version is either
Qt 4.7.4 or Qt 4.8.4.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21727 3bd1e089-480b-0410-8dfb-8563597acbee