Prepare external modules for generic use acccross projects

Hans Johnsonrequested to merge
github/fork/BRAINSia/PrepareExternalModulesForGenericUseAcccrossProjects into master

This patch set reformats the External_${extProjName}.cmake files to make it easier to share these files across different projects (ANTs, BRAINSTools, DTIPrep, DTIAtlasBuilder ....)

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