COMP: Update ExternalData module to version 1fd8d01 available in recent CMake

It includes the following fixes:

2013-02-04 Brad King ExternalData: Attach download rules to content links...
2013-01-30 Brad King ExternalData: Collapse ../ components in DATA{} paths
2013-01-30 Brad King ExternalData: Add support for SHA 1 and 2 hash algorithms
2013-01-30 Brad King ExternalData: Generalize hash algo/ext handling
2013-01-30 Brad King ExternalData: Allow DATA{} syntax to reference directories
2013-01-30 Brad King ExternalData: Allow ()-groups in series match regex
2013-01-29 Brad King ExternalData: Improve series matching using an explicit...
2013-01-28 Brad King ExternalData: Remove unused private interface
2013-01-28 Brad King ExternalData: Cleanup stray TODO and typo in comments
2013-01-28 Brad King ExternalData: Do not match directory names when resolvi...
2013-01-24 Brad King ExternalData: Remove compatibility with CMake < 2.8.5
2013-01-24 Brad King Add ExternalData module

See here for details: http://cmake.org/gitweb?p=cmake.git;a=history;f=Modules/ExternalData.cmake;h=9d84f8d135ae8b4e86d78a2e3b570a3b520445a1;hb=HEAD

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21793 3bd1e089-480b-0410-8dfb-8563597acbee