Add building of BRAINSTools as an External Project

Hans Johnson requested to merge github/fork/BRAINSia/ADDB4 into master

This is the first set of changes that allow BRAINSTools to build against slicer as an external package.

I recognize that there is still a little work to do in order to get the built versions of the tools placed into the Slicer tree and automatically recognized by slicer at run-time, but this is a significant step towards that.

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