STYLE: CMakeLists.txt: Set CMP0048 policy to NEW

Created by: jcfr

This commit allows to build Slicer setting policy CMP0048 to NEW.

Before integrating, the following PR should be integrated in their respective project:

• https://github.com/fedorov/MultiVolumeExplorer/pull/32
• https://github.com/openigtlink/OpenIGTLinkIF/pull/58
• https://github.com/SimpleITK/SlicerSimpleFilters/pull/4

Cc: @lassoan @fedorov @blowekamp @tokjun