ENH: Eliminate CMP0020 warning.

Eliminate a warning when building in Windows that relates
  to static linking of Qt executables to qtmain.lib.
This policy was introduced in CMake version 2.8.11.
CMake version 2.8.11.2 warns when the policy is not set
  and uses OLD behavior.

This change sets the behaviour to NEW as all the Qt
  executables in VTK link to qtmain.lib.

Change-Id: Ie37e447fda1af40c836da56e6c3f5d5c6ce2b4a1
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>
Merge topic 'fix_CMP0020_warning' into master

96bc743f ENH: Eliminate CMP0020 warning.