COMP: Add FindFontConfig.cmake

FindFontConfig.cmake is from VTK.

VTKv6 was not built with the module vtkRenderingFreeTypeFontConfig because
it could not find FontConfig. However, once VTK was downloaded and
configured. Any find_package(FontConfig) call (e.g. in CTK) would succeed
because FindFontConfig.cmake is now in the CMake search path.

Issue #3514

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23123 3bd1e089-480b-0410-8dfb-8563597acbee