COMP: Ensure vtk FreeTypeFontConfig component is consistently specified.

This commit fixes an inconsistency between

- External_VTKv6.cmake where the component was build only if FontConfig
was found.

and

- CMakeLists.txt where the VTK component was systematically expected.

This commit now makes sure that "find_package(FontConfig QUIET)" is
systematically used.

Co-authored-by: Dzenan Zukic <dzenan.zukic@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23769 3bd1e089-480b-0410-8dfb-8563597acbee