This commit integrates version of the ExternalData module
available in InsightSoftwareConsortium/ITK@883ecb4

Here is the details of changes added to the Slicer version:

$ git shortlog 5a11039..883ecb4 --no-merges CMake/ExternalData.cmake
Brad King (6):
      ENH: ExternalData: Do not re-stage staged object files
      ENH: ExternalData: Allow local stores without any URL templates
      ENH: ExternalData: Warn on missing file instead of failing
      ENH: ExternalData: Add support for custom download scripts
      ENH: ExternalData: Add support for custom algorithm-to-URL mapping
      DOC: ExternalData: Update documentation to match CMake copy

Matthew McCormick (4):
      ENH: Add Azure Storage as ExternalData source.
      ENH: Add ExternalData_NO_SYMLINKS option.
      ENH: Sync ExternalData.cmake with upstream CMake 3.6.2
      ENH: ExternalData multi-algorithm support

Michka Popoff (2):
      STYLE: Remove old cmake < 2.8.4 specific code
      STYLE: Remove old cmake < 2.8.5 specific code

Rolf Eike Beer (2):
      STYLE: ExternalData: Replace MATCHES with STREQUAL where possible
      STYLE: ExternalData: Clean up duplicate regex matches with CMAKE_MATCH_<n>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25546 3bd1e089-480b-0410-8dfb-8563597acbee