ENH: Sync ExternalData.cmake with upstream CMake 3.6.2
ENH: Sync ExternalData.cmake with upstream CMake 3.6.2

Change-Id: I14e5a2471f78fd0278ece2608252b52223dacbb6

stnava/ITK,thewtex/ITK,LucasGandel/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,blowekamp/ITK,vfonov/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,LucasGandel/ITK,thewtex/ITK,BRAINSia/ITK,richardbeare/ITK,hjmjohnson/ITK,vfonov/ITK,vfonov/ITK,blowekamp/ITK,vfonov/ITK,BRAINSia/ITK,stnava/ITK,spinicist/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,richardbeare/ITK,BRAINSia/ITK,PlutoniumHeart/ITK,richardbeare/ITK,fbudin69500/ITK,LucasGandel/ITK,malaterre/ITK,BRAINSia/ITK,PlutoniumHeart/ITK,fbudin69500/ITK,richardbeare/ITK,stnava/ITK,LucasGandel/ITK,malaterre/ITK,Kitware/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,PlutoniumHeart/ITK,thewtex/ITK,fbudin69500/ITK,PlutoniumHeart/ITK,hjmjohnson/ITK,fbudin69500/ITK,fbudin69500/ITK,Kitware/ITK,richardbeare/ITK,spinicist/ITK,malaterre/ITK,stnava/ITK,thewtex/ITK,blowekamp/ITK,fbudin69500/ITK,thewtex/ITK,thewtex/ITK,PlutoniumHeart/ITK,Kitware/ITK,Kitware/ITK,stnava/ITK,malaterre/ITK,Kitware/ITK,spinicist/ITK,fbudin69500/ITK,spinicist/ITK,PlutoniumHeart/ITK,malaterre/ITK,vfonov/ITK,Kitware/ITK,richardbeare/ITK,malaterre/ITK,BRAINSia/ITK,blowekamp/ITK,malaterre/ITK,fbudin69500/ITK,vfonov/ITK,PlutoniumHeart/ITK,hjmjohnson/ITK,thewtex/ITK,stnava/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,stnava/ITK,malaterre/ITK,hjmjohnson/ITK,hjmjohnson/ITK,spinicist/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,stnava/ITK,vfonov/ITK,blowekamp/ITK,vfonov/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,spinicist/ITK,blowekamp/ITK,spinicist/ITK,BRAINSia/ITK,spinicist/ITK,spinicist/ITK,stnava/ITK,richardbeare/ITK